The messaging services never opened their protocols. Trillian (& pidgin etc) has always played catch-up. It was just more difficult for the "major players" of the time because for a protocol update, you needed to release a new version of your software, and somehow have all your users seamlessly migrate to it before releasing breaking changes.

MSN/WLM, AIM, YIM, ICQ, Gadu-Gadu, Skype… all of them were reverse-engineered. The only open protocols it supported were IRC and Jabber.