Probably because a lot of PC's still only ship with VGA outputs as opposed to all Macs for the last several years that have DVI and/or ACD.
I've been looking at MacMall (ClubMac & PCMall are the same company). I've been looking primarily at Viewsonic because that 's what I already have and have had absolutely no problems with it. They are for PCs and Macs. I understand that PCs have about 95% of the market, therefore it makes since for the monitor companies to go with them instead of Macs.
If you are committed to using a KVM switch then you may have to use VGA since they are for switching VGA lines only or pay quite a bit more and get a KVM switch that switches DVI.
Ah! The price of using diverse 'machines' to do what you want! Isn't life wonderful?
Say, I'm starting to figure out how you people can quote. Quite as hassle right now, but I'll get better.