Primarily because people tend to complain only about issues. I've both funded and withdrawn from bitcoin on several occasions, no issues at all, always quick.

We're talking millions of users with significant money at stake, when things go wrong it turns to vitriol very quickly (and there is a lot of room for user error, too). You don't see others catch as much flak because nobody is even close to the size of Coinbase, but there are similar issue threads for e.g. gemini out there.