There's basically two things behind this:
1) Dell USED to use very high quality LCD panels (same or similar to what Apple uses for the ACD) in their LCDs. This has stopped in the last 6 months or so, and they are switching to the same panels most other vendors are using. I suspect that by this time next year, unless Dell goes back to the better panels, there will be a more varied responses as to which LCD to get if you don't want to spend a fortune on an ACD.
2) Price. The make a consistent product (that is you know exactly what you will be getting, for the most part - something you can't say for both the no-name brands and the big-name brands who all seem to have a massive number of options with unhelpful model numbers) for a reasonable price.
Again, now that Dell is using the 6-bit panels in pretty much all their LCDs (under 24"? Not sure if the 24 and 30" use 8-bit) I suspect their reputation for quality will begin to dip and they will see a loss in sales to lower priced brands who are using, more or less, the same components. That's kind of ironic, now that I think of it.