Robert has learned and achieved about as much as he ever would at the college level. Yes, some of that is due to the reduced targets because of Marqise's ascendancy. But it's also a fact that Robert has already set a bunch of USC and PAC-12 records. There's not much more he could do to impress scouts, coaches and GMs.
There's also the injury concern. He played on a bad ankle his entire sophomore year. He had surgery on it prior to this past season, and rumor was it was taking longer to heal than expected and he wasn't 100% for a lot of the season. I suspect he doesn't want to risk further injury that could hurt his pro career.
The third issue is Matt graduating. Wittek was decent in the ND game and pretty horrible in the bowl game. QB is a huge question mark right now, and that has to give Robert pause in terms of how it will affect the wide receivers.