I, OTOH, have indeed 80% given up on Yahoo Mail, using it only occasionally now for some specific purposes. I now deem it to be fubar indeed.
I can’t say specifically why (which is the OP’s question), but I can offer some generic observations: HTML, html-forms, browsers, everything, all have long-standing and well-developed standard ways of behaving. Things like what buttons and other controls look like, what happens when you click on something, rules about how focus moves from element to element, just about everything about how web pages look and act. This is mostly good because:
(a) Nearly all web pages have a similar look and feel, up to some point, and users get used to them and can learn how to use them, and they are predictable.
(b) The coding to implement all these things is built into browsers or into the O/S itself, and most apps use them, so they are thoroughly designed, tested, and debugged.
It gets worse: Web sites all over the place are jumping onto this bandwagon. AT&T, just to name one example, has re-designed their web site recently, such that I can no longer use it to pay my bills on-line, whereas it used to work just fine. The word is, I must now upgrade by browser, which I don’t want to do. Instead, I happen to have a Virtual Machine handy, with a browser that works. So I have to fire that up to pay that bill. Fck AT&T!
If it ain’t broke, fix it until it is!