I chose it back in the day, like 10 years ago. It worked fine so I kept using it.
I've configured it to my liking. I know its ins and outs. It's unlikely to surprise me. I've also written scripts for it, so it does what I want it to do.
Also: I remember trying Weechat once. I typed "/connect blabla" and it errored out, telling me that I can't connect to arbitrary servers without editing some configuration variable or something like that. That gave me bad vibes. Very bad decision. UX matters.