read, he said he's working on the game because it had bugs.
"ANNOUNCEMENT: I have taken the game down. I'm leaving the page up, but the downloads have been removed. Check the latest devlog for more info on this."
In the announcement:
"I've got school starting up again soon. With the work-load that'll give me, as well as another project I'd like to start working on, I'll have a lot of other stuff to keep me occupied. With that said, I'll be officially taking down Unreal PT next week. This has blown up far more than I expected, but now that it's finished, I would like to move on and start work on something original."
This does not explain, why it was taken down. It just says that it was.
I thought the same, there was no rational explanation given whatsoever, could it be copyright? malware/virus claims? could it be that he got offended somehow? I have no idea. But when you make something awesome for the world and you take it down without giving a proper explanation why... it's just unusually weird.