Different panels, & lack of demand.
Almost all current WS LCDs smaller than 24" use TN technology, which we know to be inferior in many ways.
4:3 20/21" 1600x1200 were actually what was considered "big" a few years ago, & they usually use S-IPS or S-PVA/P-MVA panels, which are much better than what in most of the new WS LCDs.
Samsung just came out w/ a TN 24" now too, & it does indeed look "good" (if you don't know what to look for), & it's cheap, which means the high end panel LCDs in the 24" camp are likely in trouble, since i know customers won't realize what they're missing out on.