Community-based enthusiasm about a wide variety of games. If you want to study a recent phenomenon, look at the social process of how Dragster infected speedrunning out of nowhere.
The speedrunning community had three critical catalysts in its growth from the beginning:
1. Quake
2. Metroid Prime
3. Mario 64
But look at how the community has evolved around these games, rather than completely fixating on them.
As an example, BBH is probably the most skilled gamer in our community, but what sets him apart as a streamer is the fact that he plays such a large number of different games. He could be half as skilled and his viewership would be roughly the same should his format remain.
At the same time, you can look at how entertaining individual games actually are from a casual viewer's standpoint. As someone who is candidly not the biggest fan of DK, I can say with certainty that watching a 3+ hour run of a highly repetitive game with nuances of skill that are only genuinely perceptible to an expert is certainly not the most exciting experience in the world.
Viewer engagement is based on a combination of interest in the streamer, and the game the streamer is playing. For the latter, the viewer needs to be able to empathize with how skilled the player is in order to remain engaged. This is where Speedrunning gets a big boost, because more people know what it feels like to be either good or bad at Super Mario Bros than Donkey Kong.
tl;dr Play other games, and let the community sort out which are the fun ones to play and watch.
