Because CRT monitors are 50 lbs and take up a huge section of your desk. Lcds are easier on your eyes if you have to stare at them 10 hours a day, you can get widescreen, and they are just plain nicer to look at. Plus you just can't really buy CRTs anymore.
LCD's use to have ghosting and color problems in the early days, but as long as you spend a decent amount of money, those aren't really issues anymore. The only exception might be for a graphic designer who needs super precise color calibration, but I bet high end LCDs will still hold their own.