Historically, there's not another position that's been demonstrably proven that you can find quality, productive players, year in, year out, at later points in the draft (and consistently even in rookie free agency) than RB. You can't say that about WR, QB, CB, et cetera, with the same rate of occurrence.
We see the same trends at other positions. For example, despite guys like Tony Romo or Kurt Warner, if you want a quality, franchise type QB, you're typically going to have to draft one in the 1st and Top 2-3 picks overall.