They would likely argue that HEVC and H.264 are hardware accelerated on nearly all of their devices whereas VP8 likely isn’t. This would mean compromising on battery life as they’d have to provide a software fallback where needed. I don’t see Apple being very willing to provide a bad user experience and given how hard they were pushing HEVC during their 2017 WWDC keynote the best bet is that they think that’s the better option.

Alternatively they’d have to add VP8 support in their chips and one suspects they would be unwilling to spend silicon on that which could otherwise be used for whatever witchcraft their silicon designers are whipping up.

I’d grant that as a valid technical reason for limited video codec support. Silicon and battery are at a premium.