Because I can't cmd-tab to it. Because it brings every Chrome window to front if I just want to read a message. Because it floats on top of Chrome whenever it's open. Because I can't easily move it between monitors, or to a different part of the screen. It's also really rubbish at uploading files. Why can't I drag and drop? And how about search? You're Google ffs. Why do I have to go to Gmail to search Hangouts, instead of allowing search in, oh I don't know, how about in Hangouts?

Some of these may have changed since I abandoned Hangouts last year for Telegram, which still has the best desktop experience.