The cost to setting up and maintaining your own server and emulating many of the more modern HipChat features (drag+drop file sharing, archived/searchable history, mobile push notifications) is not trivial. Most companies find it far cheaper to spend money on a service than roll their own in house (see also: GitHub, UserVoice, hosted WordPress, etc).
Also, nontechnical people are often not very comfortable using IRC/XMPP.