Why are 135 and 155 so much better than 145?

The divisions on either side of 145 seem to have consistently had much more talent for a long time now. Obviously Volk is an all timer and so was Max and Ilia in his brief run at 145 but the depth of talent has never been there and Volk has never really been challenged by any Featherweight other than those two since he first won the belt. Is there a reason the talent seems to end up 10 pounds lighter or 10 pounds heavier or is it just a long run of luck?