There are probably lots of contributing factors for all of the syndicated conservative columnists, but my basic theory is that it was a response to all of the "liberal media" pressure put out by the right. At some point there may have even been a little bit of truth to it. My guess is journalists of a certain generation were kinda-sorta-liberal, and these were the people who wrote columns for local papers after putting in their time on the beat. Most of them weren't all that liberal, and saw themselves as "journalists" more than "pundits" even if they were now writing columns. The point being is that maybe they were a bit liberal leaning, but they didn't see themselves in any way as connected up with a "liberal movement." But, due to right wing pressure they needed to be balanced by the stable of movement conservatives we now find ourselves with.
Of course a simpler reason is that plenty of editors and publishers aren't actually liberal and really like their Ann Coulter columns. Crazy, but amazingly true.
