Easy, TE's routinely play into their mid 30's at a relatively high level. RB's seem to fall of a cliff in their productivity right around 28 years old. So, If I'm a GM and I am staring at my 26-27 year old RB that is looking for a long term deal, Who may be great right now. The chances are that in 2-3 years his productivity is going to fall off a cliff, if he even stays healthy that long considering he has a injury history.