I'm not too familiar with the noise scene alltogether, but I think it might be possible that certain people just don't ask questions about it anymore. They just see: ok, this is the way it's done, it's normal to make limited releases, so there we go.
That, as well as the fact, that there are a lot of bands, so it's difficult to know about your own material how many people will be interested, might make for another reason for limited items. But as said, that should only count for the first couple of releases.
For a part, it might also be laizyness, people don't want to keep copying the same release for years. And if you say your items are very limited, you know they'll be sold very soon, since, as Mikko said, people will be like: I've got to buy it now or miss it. Have done that myself a couple of times, but only for releases of which I was very sure that I'd like them. Otherwise, too bad, I'll miss them. There's too much good stuff to just buy anything because it's limited.
Since I'll be doing some releases myself sooner or later and have no idea how the interest will be, it appeared to me that the best way would be to release batches. First batch of maybe 25, then see how fast they're gone and if there still is need to make another batch. That way your release stays available without having to make one copy at a time or have lots of copies still collecting dust.
On the other hand, one thing I also was amazed about: I just got the Bereft album (Your Messiah Will Fail), good stuff, but it's limited to 250 and CDr, which had me wondering: is 250 too few to let it press on CD? No idea really what the least amount is you can order with real pressed CDs.
Finally, on a side note, I had a thought about the value for money thing: it seems to be a 'problem' in Black/Death Metal as well. In my opinion a real album should last at least a half hour (I would never call anything I record an album, when it's got less than half an hour of material). But lately: Prosanctus Inferi album - 26min, Nocturnal Blood album - 25min, Black Witchery album - 22min. Why? Just write a couple more songs. Nevertheless, I bought the PI album, because it's great and will be the NB as well, since I'm confident it's better than most of the shit released. But both the NB and BW album have about 8 tracks. And I've got the impression that nowadays it's nearly not-done to have 10 tracks or more on an album (well, the PS has 13). So, then I think: it shouldn't be about the number of tracks on your album, but about the length of the album. I mean, what's next noisecore bands releasing albums with only 10 tracks or less?!
Anyway, just to say, this seems to be more of a general problem.