They were working on a Mac version, but simoroth got tired and Terry wanted to work on other games.
Give me a break. Surely it was because there was some kind of error, thus it was harder to make... no-one knows, actually.
It's a bit weird, because it wasn't really announced by Terry that he gave up. I think he thought he was still going to release 2.1. (stable), but some weeks later Simon said (in the 2.1. Beta Patch thread) that he kind of retired the project... It's too confusing for me. I wonder if I missed something...