I'm using irssi, and I guess the only answer I have is "inertia". I have an irssi setup/config which works. I could probably get weechat up and running fairly quickly, have looked at it already, but... irssi has, and continues to, serve me well, so I see no real reason to change.

Ninja-edit: didn't comprehend your question fully upon first reading. I didn't chose between them. Back in... 2007-ish? I got handed an irssi config by a buddy, and have tweaked it from there since. No active choice being made between the two. Sorry for the misinterpretation.

Why choose? I use both :-) I'm using irssi in a screen session on a server, with the proxy plugin, and weechat in my desktop as a frontend.

I like it more

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.