edit: The best way for me to explain it would be that now that something that is much better than GIF comes along, it is more likely to be used (/abused) very frequently, and we're brainstorming ways on how to not overtax every single host that this spreads to. 4chan for example imposes a hard 3MB limit with no audio. While tolerable that does limit what the format is capable of.
And believe me, this is very open to being abused, even by well-intentioned folks. Those 4 Sonic webms I posted, for example, are targeted at 15Mbps (on purpose, for me to test out) which is way more hefty than a video of those dimensions and complexity warrants. They could easily have been something akin to 3Mbps with no discernible loss in quality, even at 60FPS.
Bandwidth is going to be a really significant factor when this starts getting used a lot more than it is now. Especially on mobile. Especially on GIF thread quote pyramids. (Yeah I know, who browses GIF threads on mobile anyway, etc. etc.)