1920 x 1200 was a great resolution that filled a nice niche between the crazy expensive 2560x1600 monitors, and full 1080p HD monitors quite nicely. The added vertical pixels make all the difference when working with web pages and spreadsheets, and is a much nicer view for games too. It looked like a year ago, this was going to become the new mid-level standard. Now all these monitors have all but evaporated from the marketplace.
1080p is very nice for games, but it gives me a bit of a claustrophobic feeling because of the very narrow (16:9) aspect ratio. IMHO, 16:10, like the 1920x1200 monitor, is much more natural to look at as far as field of vision...
I suppose it has to do with economics. Let's assume 23" 1920x 1080 vs 24" 1920 x 1200 for this arguement. More 1920 x 1080 panels can be cut from the same sheet of "LCD glass" than 1920 x 1200 panels. This effectively means more LCD panels can be cut from the same piece of glass. Thus, cost per unit decreases as the number of units increase with all other things remaining equal.
This results in a typcial 1920 x 1080 monitor costing less than a 1920 x 1200 monitor. The consumer drives the market and as usual they want the best bang for the buck. The result? 16:9 monitors have become more popular than 16:10 monitors.