I throw darts at a dartboard and see where they hit...
There's a lot that I consider before accepting games into a marathon. If I had to break it into three factors, they would be popularity, entertainment, and donation potential.
There's no denying that a game's popularity is important. I do say the term, "too obscure" constantly when I reject games, but what defines popularity? I would break it down into three types of popularity.
Sales is ironically, the least important of the popularity categories. Sales can be a decent rule of thumb for determining whether a game will be successful at marathons. After all, Mario, Zelda, Pokémon, and some Final Fantasy games have all sold millions of copies. However, there are other best-selling series such as Madden and Gran Turismo that would flop at a marathon. Madden and Gran Turismo games might be fun to play, but Madden can't be speedrun, and a Gran Turismo speedrun would be quite boring. Looking at a car drive around isn't exactly the most thrilling thing to watch.
Basing choices around sales is something I did for the first Awesome Games Done Quick back in 2011, and I definitely made some big misfires in game choice because I ignored other facets of popularity, and whether the game was an entertaining watch. I won't name any names, but there were definitely some misfires.
Community is important, because if no one in the speedrunning community likes the game, then that means there will be no interest in the game. After all, if the runners like the game, then they'll be more likely to watch the game at the marathon and people will be enthusiastic about the run.
There are some flaws to the community approach. First, the speedrunning community is growing bigger and bigger, so defining the community can be tough, especially since there are multiple speedrunning communities. Also, certain games, such as JRPGs, definitely have dedicated communities, but there are also a lot of people who are indifferent and/or don't like them, so getting a consensus would be difficult, if not impossible.
Also, sometimes speedrunning communities will play games that are very obscure and don't really stick out to a general audience, which is a big reason why you can't let speedrunning communities decide every game, and also highlights the importance of the final type of popularity.
Fondness and nostalgia is the most important type of popularity. If the people watching the marathon don't like the game they're watching, then they'll lose interest. Also, if viewers don't know the game and there isn't something impressive about the game they're watching, that will also make them tune out and lose interest.
Nostalgia is part of the reason why Nintendo franchise games are so prevalent in marathons. People loved playing Mario, Zelda, and Metroid games in the past, and they have fond memories of the games. Yes, this includes Final Fantasy and other JRPGs, while there are definitely a lot of people who don't like JRPGs, donations and comments from people watching have said that there are a lot of people who do love JRPGs and are still fond of them to this day.
The save or kill the animals donation incentive is so effective in Super Metroid because Super Metroid is one of the most beloved games of all time. It's in numerous top 10 Super Nintendo game lists and usually near the top (if not at the top) of best games of all time lists. It doesn't hurt that the game also has a very active speedrun and racing community.
A good example of a game that (some) people are fond of despite not selling a lot is Megamari. Megamari is a doujin game (aka fan game) that uses the characters from ZUN's Touhou series for a Mega Man-style game. Needless to say, there really isn't any sales data on this game, and it's safe to say it's not a million seller. Does everyone like Touhou games? No, but the Touhou games definitely have a dedicated fanbase, and the game raised over four thousand dollars in about 45 minutes (when you include setup).
This is a very hard type of popularity to define, but it's the reason why something like Megamari can get more viewers and perform better donations-wise than a game like Mass Effect, which has sold over a million copies in just three weeks.
The real kicker here is how "watchable" the run is. Ninja Five-O is going to be in AGDQ 2014, and it's hardly what I would call a popular game, and as far as I know, it didn't sell very many copies, but it's a very fast-paced, has a lot of interesting movement and is a good watch. Also, Super Metroid not only has fast-paced movement, but there's also many tricks and glitches involved in a good run, it's not only popular, but it's also a fun watch.
Now not every game is entertaining because it has fast-paced movement. Zelda games generally don't have very fast movement, but there's a wide variety of glitches that have a wow factor to them. There's also Hotline Miami, which doesn't have a lot of tricks, or even advanced movement (you can only walk around), but there's always a lot of action happening in the game, which I think is one of the reasons why it was successful at SGDQ 2013.
Obviously, entertainment is subjective. Some people find JRPGs such as Final Fantasy a great watch, but many people do not, despite how well they do donation-wise. There are even people who are tired of watching the two games that have been in every marathon, Castlevania: Symphony of the Night, and Super Metroid. While it is a small minority of people who are getting tired of those two games, they do exist. As I mentioned in my last post, it's good to have variety to cover different tastes and avoid stagnation, and what people find entertaining is no exception.
There's no denying that raising money is a big part of the marathon, and one thing that helps is the donation incentives attached to a game.
One reason why Final Fantasy games are successful donations-wise is that they have a lot of donation incentives attached to them. You can name the characters, and there's usually some in-game incentives too, such as who Cloud can date in the Gold Saucer, and who sings the Opera in Final Fantasy VI. Almost all of the incentives for Final Fantasy games are successful and raise a large amount of money.
However, games don't always need a lot of donation incentives to draw in donations, If the incentive is powerful, then a game only needs one donation incentive. The best example is killing or saving the animals in Super Metroid. This incentive alone raised over $30,000 during SGDQ 2013, which is over 10% of the money raised! I think a large part of the appeal is that the incentive is so direct and appeals to simple instincts (be good and protect animals or be evil and let them burn for a good speedrun). It also doesn't hurt that saving the animals is canon in Metroid, and video game fans are very picky about canon.
Now notice how both incentive examples I used are from very popular series or games. For donation incentives to be effective, people need to know the game, which is why popularity is important.
If the game has an amazing prize, that also helps. Prizes like this Zelda shield and this gravity suit perler are both super awesome, and are big donation draws. It's been proven time and time again, marathon after marathon, that games with good prizes get more donations than games without good prizes. Even games without strong donation incentives, can suddenly have one with a good prize, which I think is a big reason why MegaMari raised so much money, it had a cute Touhou doll prize.
Now there are very few games that hit all three categories, popularity, entertainment, and donation potential (the list is probably less than 10 games), but that's why there's a mix of games at the marathon, so all of the bases are covered, and there's some variety for everyone.