I think it was a number of things that happened in a pretty small window of time:
Virtual pinball - The success of games like Pinball Arcade. I think that this being released on phones is particularly important.
Generational - Those that grew up in arcades are now in their peak earning years. Disposable income + nostalgia. These people are seeking things to fill their homes.
Documentaries - Tilt, and Special When Lit are both fairly popular, even outside of the community (they are or were readily available on netflix).
New Manufacturers - JJP entering the arena has created quite a lot of buzz. Other boutique manufacturers have also popped up.
Better games - I'm not necessarily saying better than the 90's, but Stern has responded to the above by releasing some of the best games in years.
