Approximately nobody used the federation capabilities because approximately nobody used Google Talk... LOL
In more seriousness, I know of quite a lot of people who chose to use Google Talk specifically because they could use it with their client of choice. (It also had a very wide reach for a while, thanks to its Gmail integration edit: and federation)
I honestly don't know anyone else who used Google Talk (other than whenever they checked their Gmail they'd also see any messages in it).
In other words, Google killed off the two reasons why people used it (edit: reach and choice of client). Since then they've "replaced" pieces of it with multiple different apps (Hangouts, Chat, Allo, Duo, Meet), some of which they're finally deciding to merge back together, and in the process all they've ever done (and all anyone expects them to do with this merger) is remove features. Usually exactly those features which were the only differentiation those services had from the competitors. Which means there's no difference from a competitor with a wider reach (Zoom, 100 different IM apps, iMessage/FaceTime, etc.). Which means it's doomed to be terminated (and probably reincarnated) by Google again in a few years.
(Maybe there was even a third reason to use Talk: it was the IM with Google accounts. I don't think there's been One IM To Rule Them All from Google at any point since then.)