Because running a likely 4.5-4.6 as a 5'9 RB is not going to look great.
Emmitt Smith.....
Never liked him...but that's almost exactly what he did at his NFL combine...
and JK is no Emmitt Smith--but the point is that 5'9" RBs who run around 4.5-4.6 are absolutely fine for the NFL...if they are durable and tough enough.....
I don't see JK ever being durable enough to make it to a 2nd NFL contract....