Luck has performed very well by rookie standards, but average by NFL standards. RGIII is playing on the same level as the NFL heavyweights. So while they've both done well, they aren't close enough for co-award talk.
One thing I was thinking about was what if these guys, instead of being rookies, were in their 4th year? Obviously they're not, but humor me for a minute.
RGIII would viewed as a solid franchise QB, not gaudy, but hyper-efficient. Luck would have questions around him, as would any QB with a 55% completion rate that had 16 turnovers in 12 weeks. He'd be keeping his job because they're winning, but when wins started becoming scarce, he'd be in trouble (Sanchez with Jets got away with mid 70s ratings while they were winning, Vick with Eagles has a similar stat line to Luck and yet was getting questions and was probably on his way out even before his injury).