It was easier for us to build and maintain back then. We had push-back from some of the companies (Yahoo being notoriously aggressive about it, to the point of actively banning our developers' accounts and inventing new obfuscated auth schemes all the time), but from a technical standpoint:
1. Most of these were communicating over plain text channels (making it easy to sniff traffic from official clients)
2. The protocols were generally understandable (MSN for a long time was largely all MIME-based messages, and could be understood by watching a traffic stream)
3. Used simple usernames/passwords for authentication (just fewer hoops to jump through)
It's a different world nowadays, with more focus on keeping you in the chat environment and offering additional features as part of that experience. Harder to abstract a lot of that out.
That said, Gary Kramlich's been working on this diligently. (If interested, he also regularly streams development over at https://www.twitch.tv/rw_grim).