I'd guess that the reasons are lack of competition, standardization, and/or volume. I got a crappy tornado projector for the kids, and it takes standard track lighting bulbs. It's really dark, but I got a two-pack of xenon replacement bulbs for $8, so if xenon is the only rare material in the fancier bulbs, that's likely not the reason for high prices.

The argument about precision in manufacturing is interesting, but it seems that hotspots could be fixed in software. I've seen projectors that calibrate other things with structured light. Correcting for hotspots should be relatively easy, especially if you consider the electronics basically worthless, and doing so would let you use much cheaper bulbs.

I'm sure there are lots of ways to reduce prices, but it's likely not advantageous for the producers to do so, and nobody else is stepping in offering generic replacements, so maybe the market is still too small and fragmented.

And I'll bet it'll stay small as long as it costs on the order of $1-$2 per hour just to run your projector.