You can find his talk about it a bit on Twitter and in the Github issues. But as far as I remember it mostly boils down to the fact that thing/geom got very little traction and external contributions. He developed this huge ecosystem on his own to make digital art but it wasn't being really picked up by others. Digital art is typically quite collaborative with people giving workshops and stuff. The switch to TS was so that other's would join up and develop the library together.

Note that part of the problem was that the library was written in a highly unusual literate style. So you had to clone the repo, then use Emac's Orgmode to tangle it to get the Clojure source code. This created a lot of friction for people to contribute.

Also before deps.edn (Clojure built-in dependency management) was added to the language you had to use leiningen which didn't make using a locally cloned fork as easy as it is now (now you just point to the library file directory and it "just works")

> Or are there other reasons why this lib was moved to TS?

I'm pretty sure reach was the motivation. The author basically wrote his own TypeScript libraries duplicating a bunch of Clojure stuff, so the code is quite Clojure-like still.