Dragonriding is almost 3 times as fast, that’s why. If regular flying was as fast as Dragonriding you’d never see me on a Dragonriding mount again. I (and I think many others) had assumed regular flying would be similar speed when we asked for it.
I use dragonriding when covering long distances quickly and standard flying for everything else. Each has its own niche. Both types are available, why wouldn’t you use them both?
This is a big part of it. If hovering were possible with dragonriding I’d probably use it a lot more. Sometimes when you’re approaching an area you need a second to evaluate the best place to land and you just can’t do that at mach 2 with your tail on fire.