To some extent, it's self-reinforcing. Once the FireTV gets a lead, all it has to do to maintain/extend that lead is reasonably support playback of whatever new format/source and Plex works great on it. If FireTV supported TV as well as TiVo does, it might end up with 100% of the living room display share.

(I also have Plex sharing to devices outside the house, but that's a <1% use case, mostly when it's us traveling somewhere and the kids wanting to watch something that's on Plex.)