Sometimes the kid can't gain anything by coming back, and can actually lose draft position.
DC showed such a small sample of what he can do, that he could easily take his draft stock from 20-30 to 5-15.
Honestly, it is often said that guys are better off improving in an NBA system with all the resources that they have available to them...
Logically that makes sense, except it doesn't happen nearly as often as you think it would. Not sure why, probably a mix of the franchise not being good enough at development (so many are lousy at this), the kid's motivation (it can be a real reality check to be sent to the g league, or on the bench for 2 years, some kids lose motivation), and the money changing things (even a back end 1st round contract is insane money that can disrupt the way a young man thinks about things[Thabeet]).
DH has shown us time and again that players develop in his system. This is a good choice for DC, imo.