My (non-physicist) understanding of this, is that neutrinos from then will be quite low energy now, due to the expansion of the universe(?), and so not detectable by the method IceCube uses. There is a minimum energy required, in part because the charged particles generated by the neutrino interaction need to be moving faster than the speed of light in water (or whatever the detection medium is) for there to be Cherenkov radiation generated and then detected.

That said, I would love to see a response from a more knowledgeable person!