Cos they tend to have higher quality panels. The cheap ones tend to have TN panels which are 6bit and have questionable color reproduction. However there are LCDs with DVI input and TN panels. The real necessity of DVI for TN panels is a topic of debate. Higher end MVA Overdrive, PVA and S-IPS panels tend to carry DVI as they are a lot more accurate and sharp and obviously cost more to produce. These are all 8bit panels.
