If you're talking about the functionality, yahoo fücked it up years ago with progressive steps "redesigning the website" that took it from perfection functional to something that barely works and often fails.
If you're talking about the quality of the users and their answers, the primary reason it's gone to the gutter is one single setting "Make Anonymous". The site used to have its fair share of trolls, but once they could hide their already-anonymous screen names behind another layer of anonymity like little chickensh!ts, it's brought out the childishness and trolling a thousand times more. And with yahoo doing almost nothing to respond to complaints about abusers of the site, the trolls feel they can get away with posting anything and behaving however they want. I've been using this site for close to a decade now and have posted thousands and thousands of answers, and never once have I set any answer or question I've written to anonymous. Whether writing a long detailed answer or telling some racist to fück off, I have the integrity to put my name behind it every time.