If I had to hazard a guess, it'd be that HTML email is probably one of the lowest priority things on the roadmap over at Google or Microsoft. The vast majority of regular folks sending emails send mostly plain text, and even many company sent emails also on the basic side (though with these limitations, it's probably self reinforcing there).

Add this how more complex emails could potentially lead to security issues for their webmail clients and waht not...

And well, "eh, good enough" is probably the order of the day.