And you assumed everyone who wanted a desktop chat app used pidgin and nothing else? Because there were plenty of "multi-service" apps. It's just that Pidgin was the more popular among Linux users.
Yeah, Trillian was, by far, most common among the general public (the non-programming, non-techy types). Honestly, Pidgin was pretty niche, I don't know of any non-techies that used it.
Multi-chat clients had limited appeal by design because non-techies were more likely to just pick whatever service was the most convenient to them. Seems expected that it would be the techie folks consolidating all their friends because they were more willing to use multiple services than the average non-tech user. Even Trillian was fairly niche by comparison, just far less so than Pidgin.
Pidgin's psychic mode plugin was great fun, though.