You seem to be too young to have been around at the time, or you didn't pay attention back then.
Google was a small upstart at the time, riding to success on the back of open protocols like XMPP (outside of their core search engine business) that had all the support from tech enthusiasts back then*
Google's XMPP server was simply the easiest one to switch to from ICQ/AIM/MSN which loads of people did back then.
Only when Google had pretty much absorbed all potential users did they decide to not really care about open protocols after all and stopped developing the s2s federation support.
*Edit: this conversation played out many times back then: "did you hear of this cool new messenger called Jabber?” - " No, but cool, where can I sign up for it? ICQ sucks" - "hmm, do you know Gmail?" - "yes, I just signed up there because the awesome free 1GB email space” - " ok, cool! Then you already have a Jabber account, let's use that one :)"