Money. It’s about money. Running backs want it. Some of them do more than enough to earn it - looking at you, Jonathan Taylor. But the NFL operates under a salary cap system. The system's limits put general managers in a challenging position as they manage their team’s annual budget.
It has become common for owners and general managers to use the money at other positions, allowing running backs to leave for other teams, franchise tagging to reduce cost, or offering less than they would for players at other positions. There is a football reason for this, a strategic one, but let’s focus on the money.