1) Mobile CPU/GPU is still 50X slower than desktop (i.e. your CSS transitions are 50X slower...hardware acceleration included).
2) Mobile RAM allowance is still 10X lower than desktop (i.e. less resources loaded, prefetching, more thrashing, less JITed code, less unboxing, etc...).
3) No native look and feel, so everything seems out of place.
4) Still takes 3 years to get anything through the JS committee, and god knows how long before browser adoption.
5) Users still can feel latency even if backend is optimized (queue post about X% drop in latency equates to Y% increase in app usage/engagement).
6) Did you know that FB was originally all HTML5 on mobile? Did you know solely because of that decision, they were at most two quarters away from going belly up -- until they shipped their mobile app which turned everything around?
The fact that you brought up desktop being your primary client goes to show how little regard you have for the problems on the mobile platform.