Probably some combination of clicks, potential lack of mobility, and an effort to not let a high initial ranking (which is always a projection that includes growth that hasn't happened yet) continue to dictate the final rankings once they have the full high school career to look at. They specifically call out his -92 rushing yards as a senior in high school being incredibly low, even for a "pocket passer."On the last point, I do think there's some merit there. Some guys are freaks as underclassmen so they get ranked initially, and if they were maxed out and don't get any better as a junior/senior they probably shouldn't maintain that ranking.
But realistically all prospects are risky, and being low on a 5 star QB going to school that has never had one is probably smart. If he is a stud, Nebraska fans will let them know about it for a really long time but that's it - it's not like they have him outside the top 100. And if Nebraska can't develop him effectively and he stalls out as a decent college player, it's a feather in their cap showing they know more than everyone else. Recruiting sites figured out quite a while ago that it is smart to factor in where a kid is going (or at least who has offered him) on the OL where very few people are equipped to talk about good versus great prospects. Just don't usually see it at other positions like QB, but I think it is likely one of the factors.