1) Personal preferences: different terminals come with different defaults, some people do not want to tinker with the settings, so they just stick with whatever immediately clicks.

2) Technical curiosity: some people probably started their own terminal project just for the fuck of having made their own - and they eventually gained a userbase.

3) Feature set: Most terminals implement most features these days, but at the time these projects started, you'd have a feature matrix between features and terminals because most terminals lacked some features; so people would pick whatever suited their needs.

Personally I've been using kitty for years mostly due to 3), at the time I chose my forever terminal, I evaluated like 12 different ones, and kitty was the only one that covered most of my needs without feeling like it suffers from random quirks, and I like that it has built-in terminal multiplexer (a'la tmux) and emoji picker.