it's fascinating, and it has to be more than that the running game has been deemphasized over the last 15 years.
Is it because running the ball is less of an actual skill/proficiency than catching the ball or throwing the ball so there's a greater supply of NFL capable RBs (vs. NFL capable WRs or QBs).
Is it because running QBs are so fashionable that they're taking away some of the normal RB's roster value?
Maybe that running the ball is so punishing that fewer superstar athletes are going that route in HS and college (instead they play QB or maybe defense) and so there are less "game changing" RBs that would command big salaries?
One also has to wonder if the trend will eventually reverse.