Some things here:
First, the "experts" who draft athletes for the pros are often doofi who have no clear idea of what they are doing. Example = several pro teams want to see Lamar (!) Jackson (!!) try out at WR (!!!). Why? Well, the easiest answer is that unless a QB is tall, strong, and has a terrific arm (being white helps too) there are pro scouts who don't think they are good prospects. A lot of these people didn't want to draft Russell Wilson or Drew Brees for these "reasons". In short, the Moneyball scouting revolution hasn't hit the pros yet.
Second, a trip offense has problems attracting interest in its players. You'd think that Shaq Mason and the WRs would have changed some minds on that, but apparently not.
Third, and this, imho, is the biggest barrier, many of the best players Tech has had recently have not been playing in positions where the pros would like them. Example: Simmons. He was a rush end and not even a starter. In the pros, if he makes it, he'll probably be at OLB. (I thought he should have been there from the first, but there it is.) Soooooo … you have a team that didn't have a winning record with several prospects that you will have to switch into new positions. This automatically means that taking them in the draft is probably not worth the risk, especially since none of them went to the big tryout.
Fourth, the senior class was small for the second straight year and all the tryouts were at Tech's pro day. That limits the comparative data available to the scouts.
But look on the bright side. If Mills had stayed, we'd all be on tether hooks about him leaving early. And some of the guys in as free agents will stick. Count on that.