I'd guess you see fewer 91/135 pilots in a major airline newhire class compared to regional guys simply because there are fewer corp/charter types that presently want into that segment of the industry, whereas for regional pilots it is the most logical step up on the career ladder.

Its easier for airline pilots to network with other airline pilots, just like it is easier for corporate/135 pilots to network with corporate/135 pilots. While its difficult to break from one segment into another without good contacts, my experience has told me it is MUCH more difficult to go from 121 to 91 without strong networking than it is from 91/135 to 121.

Bottom line is no matter where you work (121 regional, 121 major, 91K/fractional, 135 or pure 91) make sure you network all segments of the industry and keep those relationships active!