in my humble opinion the major reason is because people don't want to adapt to doing something different. that and people have never historically used steam groups in mass probably for much the same reason they have to do something different than they already are doing. TS3, mumble, Vent, irc, etc...
if they made it so steam chat was integrated into the game like coh1 people would probably use it and not even relize that they were actually using steam groups.

heck everyone who uses any kind of chat twitch, steam, probably coh1, etc... all it is a fancy irc interface its nothing special.
the main problem is that i never used chats in the first place. i used the steam chat system or anything other than vcoh mass clusterfuck that was the chatrooms