When I first signed up I thought "Oh well, it must be difficult to have all of them configured at the same time, so enough with PHP", but I got my server and I really think it was easy to configure all of them, even Python with both Publisher and PSP. Is there another reason than rather just the complexity? I got that also for SQLite and PostgreSQL (although I would really understand why no more than just MySQL, for it takes a huge part of the server load, at least under mine configuration) configured to work if I ever need them to run (they are not running right now).
JSP is terrible and causes a massive load (eww. Java). I believe python works, on one of the servers at least. Ruby requires SSH access, and is therefore not an option.
I would consider Java to be a completely different kind of hosting actually. The same for asp/.NET and also ColdFusion.
The main reason, though, why we don't support all those things is indeed mostly complexity. They are rarely requested and it's not easy to keep everything working especially if I'm not familiar with it.
Python should normally be working, though. I've been trying to get Ruby (On Rails) to work but it didn't work completely so I left it as a possibility for the future.