I picture 2 reasons for why it doesn't happen more often.
1. Most of the time, you see 3 or more babies on a stalk, meaning each one gets less paste individually.
2. Except in cases of higher then normal desire to 'take it easy', usually seen in 'shithead' tier yukkuris, the parents probably wake up when the babies starts moving/making noise and prepare a soft place for the babies to land. More forward thinking yukkuris may actually insure the 'mother' sleeps with the stalk over the 'soft place', so the babies will be fine, even if the parents sleep through the birth.
I think the baby died primarily because it was the ONLY one on the stalk, so it got TOO much bean paste, and as such was fatter then normal. This caused it to fall fast enough that its skin couldn't take the landing, therefore, *SPLAT*. The fact that the yukkuris didn't have a soft place for the baby prepared, and the father apparently wasn't willing to use its hat, cause me to think they might be 'shitheads' as well. The mother sleeping through the whole thing just adds more credence to that as well, in my mind.
Usually yukkuris parents are surprisingly genre savvy enough with the kids birth, not always of course, sometimes it all reduces to luck and the quantity of the kids, i do recall that story of the big yukkuri family that after getting to many kids decided to leave for a new easy place but in their trek they got systematically wiped out, in the beginning they were pretty much launching babies from the mammalian pregnancy without any worry to the point we could see one slammed dead against the wall.
Oddly enough this reminds me of waterballoons, used to love testing out how high I could drop it from before it would burst...
Quick fun translation for the hell of it, even though it's quite obvious what's going on; 1st panel koreimu: yu, ...yu,! [sfx around koreimu] *puru**puru**puru* (wiggle / sound of shaking) 2nd panel koreimu: bweing bwo-- (being born easy, slurred a bit, start of sentence) [sfx]: *bote-* (splat / sound of meaty wet impact) 3rd panel koreimu: --rn eajpiyau.. (being born easy, rest of sentence, interrupted end) [sfx] *buchyaa-* (splash / sound of a nice plump waterballoon after hitting the hard concrete floor) 4th panel mother reimu taking it easy: *ℤ ℤ ℤ* (snore / sound of blissful sleeping)
Nicely explains why this might not be "common". I also think that for a successful ko, it makes sense that they inherited good instincts such that they have successful births themselves.