For a long time, I've wondered why this application is not used by 99% LaTeX users.

Is it the UI?

Is there a good alternative?

Besides IPE, there is also Asymptote. (https://asymptote.sourceforge.io/)

I think the lack of uptake is due to inertia. It takes a lot of effort to get up to speed with any system, so you are most likely to start with and stay with whatever people you know are already using, which seems to usually be TikZ.

For just the occasional illustration, it's easier to make do with something like Inkscape. (https://castel.dev/post/lecture-notes-2/)

I have never heard of it. It also does a poor job of marketing as I have no idea what it even is from the documentation (https://ipe.otfried.org/manual/onepage.html)

Whenever I needed diagrams for my latex documents, I usually just use tikz