It's because a bunch die in the drifting stages, because food has to basically fall against them. Then, in the next stages, they've been observed to be very cannibalistic - which I think is more because of the very sterile environment that they've been raised in in captivity, than because of natural inclination.
Then, when they are making the transition to land, in captivity, a great many of them drown, because they are raised in jars, there's no slope. People who have raised them have been setting alarms for every 2 hours to try to catch them at the exact right moment that they start to transition to move them, to not lose as many.
People are afraid to lose them to a non-sterile environment if they raise them in live water on a sloped, sanded (aka, impossible to keep sterile) tank, but I feel like there's a lot that can be learned from trying, especially as getting a handful of babies from 1000+ eggs is a success with the current methods.