4 reasons:
1. There are 32 full time starters at QB in the NFL. There are only a dozen or so featured RBs who get the lion's share of touches for their team and the rest split touches to a greater degree.
2. Most leagues start only 1 QB but they start 2 or 3 RBs.
3. RBs are more injury prone.
The first reason impacts the supply and the second and third reasons impact the demand. All three of them combine to make RB far more valuable than QB absent scoring differentials that change the picture.
4. But scoring differentials don't really change the picture because the drop off in scoring from the 1st QB to the 12th QB is usually far less than the drop off from the 1st RB to the 24th or 36th RB.