Same thing that happened with Nick Lodolo (41st overall in 2016 didn't sign and went to TCU), he wants more than the 5% overslot the Pirates are willing to give (which is the magic number to avoid any penalties).
Lodolo, by the way, is shaping up to be a Top 10-15 pick next year.
Good news, though, the Pirates will get the 37th pick next year as compensation... so, yippie?