The problem with doing a desert or snow MA map is that to make it look any good you would need to add your own ground textures. While there are beach and snow textures in the default terrain ETO and PTO sets, you'd really need a lot more variety than one snow or beach texture. MA map designers are allowed to add ground textures to MA terrains but these textures tend to make the resultant terrain file huge. HTC don't like massive MA terrains, probably because it takes players with low speed connections too long to download them and also bandwidth for them = money.
To look really good a desert or snow terrain would also need revised objects, i.e. snow covered trees and buildings or North African style mud buildings in the towns. However MA map creators are not allowed to change terrain objects in this way, since to do so could introduce bugs that could crash the server or players' FEs.
