Seems they're being conservative with promotions, but I think at Clearwater down, it's a matter of how much time they want kids to have with instruction v playing games. Right now they can spend a lot of time with prospects, after the draft when new players come in, they may then aggressively promote and clear the board for the next group.
The other factor is a 1-2 year "hole" where the roster limits stateside slow the promotion of DSL players and thus players available for GCL and Clearwater this year . As DSL players get an extra year, they will be in "up or out" mode, with more time to evaluate, the ones sent stateside will be promoted more aggressively.