Actually it isn't.
RB who are tall have a harder time getting their pads low enough to win leverage on contact. There haven't been that many highly productive RB over 6'2" I think RB who are taller than this are at a disadvantage compared to a shorter more compact frame.
Derrick Henry for example has length working against him.
There have been some great RB who were tall of course, but looking at the overall population of successful players, the majority of them are 2 inches from the ideal of 5'10"
I think there have been more successful RB who were shorter than 5'10" than there have been successful RB significantly taller than this.
@ZWK did a study on this several years ago and he found correlation between weight and RB success but not height. Other metric studies I have looked at identify the same thing.