It is the shelf life of a RB that makes sense going late in the draft helps. These guys take a beating, lasting usually 3 years. Would a 1st or even a 2nd make sense, unless they are elite, if you can get an adequate player in the 4th or 5th who will help for the next 3-4 seasons? Get a couple guys who can play off each other and you don't need an every down back.
You can groom them as well for a couple seasons and not put milage on them. It's different than a QB or premium positions. If they are BPA, should take them anyway, but if it is need, make a later selection. WR, QB, and OT are premium on offense. DE/OLB, MLB, and CB are defense premium. If they are BPA at the draft early, take one of those positions. Many other positions can be filled later in the draft by organization philosophy and type of player prefered. TE, RB, DT/NT, S, OG, C, OLB(4-3 only) can be found later IMO. Unless BPA, draft the other positions first.