It comes down to role definition. The Spurs have an incredible system that carves out clear-cut roles. That way a guy like Kawhi Leonard is only responsible for rebounding, defending, and hitting the open three, and he can focus all of his energy there. Ross would look great in a Spurs uni (then again who wouldn't), and it's because he could focus strictly on doing one or two things really well (say defending and hitting the three), versus just going out there and being told to play. Like we're seeing Ross try to handle the ball and create for himself, things he can't do, and it's obvious that right now he's just way over his head.
The Spurs' system is perfect given where they draft every year. Guys in the late first round and early second round usually only have one or two high level skills anyways, so it makes it easier for them to swallow when they're told they're only going to be shooting the three, setting screens, etc. It's just great for a rookie who isn't sure what type of player he's going to be.