It happens like that sometimes, I can't guess why. Sometimes I can scrape up hundreds of eggs and it seems like they all hatch. Then as they come along they get fewer and fewer until when they're big enough to let out, there's only a handful left. Sometimes I get a much smaller batch of eggs and it seems like they almost all make it to a good size to let them loose. I don't think it's anything I do differently, I try to treat every batch of eggs I scrape up the same. As often as they lay in my tank, it's probably for the best that only some of them make it to adulthood. I still usually have plenty to give away to friends and family.
You'll probably be in the same position I suspect. Even if you only get 3 to adulthood from each batch, that's still going to give you a good supply of them for the foreseeable future. I started with only 3 of them about 12 years ago and I've never been without a good size "school" of them since. One of the parents long ago was an albino and I still get an occasional one to show up. I think I may get another couple of them to bring some new bloodline into my tank. After all this time the gene pool must be getting kind of weak.