Old Sites = more links because webmasters are lazy to take them down
New Sites = webmasters who refuse to add links due to time and effort - too many requests
Webmasters are just plain tired of adding links where in the past they were more likely to be willing to add. I'm not saying that links don't work...they do. I'm just saying that the older sites have too much advantage because back in the day it was easier to get a link...they live off the past links.
Another thought...Google needs to randomize search results more. If Google mixes up search results more then their users will see more variety. Variety is the spice of life.