Because how he produced is college, lots of deep throws and taking off if your first read is covered, won't be as efficient in the NFL.
He was typically the 1st or 2nd best athlete on the field in college, and the college game is designed to take advantage of that. The NFL, where there is a much smaller gap athletically, is more about scheme.
JD hasn't shown as much in college that will directly translate to the NFL, therefore it is more of a project on how he will fit, and the more projection needed the lower the floor is.