Probably because they don't want to have to figure out all the various overscan/underscan checkboxes in various video card drivers.

I kid, but in reality, supporting features is expensive. They probably figured that the number of calls they'd get would be more expensive than the number of people that woudn't buy the set because they couldn't hook it up to their PC through DVI.

That, or they were confused about the interplay of HDMI, HDCP and DVI.