Because I don't need to. I've never even once thought "my terminal feels slow and I want to find a different one". My terminal (xfce4-terminal) also uses a GUI toolkit that makes it visually fit in my desktop, and I like that.

Regardless, sometimes you just gotta write some code because there's an itch that needs to be scratched. Sounds like this author really wanted to see if he could write a super fast terminal, and make one faster than the incumbents. It's a cool achievement!