Luke was one of the only other Panthers players other than Cam that the overall NFL fanbase knew (and almost certainly respected more). He will get in one day.
Regarding Sproles, not diminishing his accomplishments because he proved that even smaller players can contribute to a team. The issue he'll have is that he was more often a compliment to the offense rather than a feature. He had flashes of greatness but not enough to stick in people's memories. I suppose a litmus test will be Devin Hester because if he can't get in, then Sproles won't have a chance.