Just curious... do you actually have defensive players... not teams... players?
Like JJ Watt or AJ Hawk?

Everybody is talking about defensive teams (most common) but I believe there are pools with defensive players (that count tackles, sacks, FF, etc... for individuals).

Anyways, RB are scarce in any league because they are the goalies of their sport.
There are only 32 #1 RBs (to go with the 32 teams).
And in most fantasy leagues, teams start 2RB... so in a 16-team league, every team might have 2 #1 RBs.
This means that no team even has a 3rd #1 RB... which makes them scarce (and THAT is the value-spike they get in trade-discussions).
Any back-up RB should have a huge statistical drop-off.
In other words, maybe 32 #1 RBs have a 200+pt value (say)... then the best back-up RB might be a 120pt value. Huge gap.

WRs rotate in and out... QBs are mostly a start 1, so they're always there, same with TEs.
If you have defensive "players", well since each team has 3-4 starting LBs, 3-4 starting linemen, 4 starting secondary... there's just a lot to pick from.

Scarcity of RBs = scarcity of goalies in hockey.
Treat them a bit the same way.