There's one explanation for MM's results which has nothing to do with a nefarious conservative cabal running the newspaper industry.
Demographically, newspaper readers tend to be older than non-newspaper readers. An older audience is likely to be more conservative. Newspapers are generally in business to be profitable. That means, more often than not, providing consumers with products that reflect their tastes. Thus, more conservative syndicated columnists than not.
But, hey, if you want to hasten the extinction of your business model be my guest.