Another possible contributing factor: it was in 2011 that updates during the comp were allowed for the first time (and wow, there was quite a controversy about that at the time!). Maybe it made many buggy or unplayable games substantially better, especially for parser games? (I don’t know if you’ve played the updated games or the games as they were first released.)
Admittedly, 2011 is four years before 2015. But maybe that new option took some time to be widely adopted?