That limitations can improve a competition does not mean that all limitations improve the competition.
Limiting that which outwardly is most important in making things look more varied is a surefire way of actually killing creativity.
Imposing limits on brushcount, entitycount, modelcount, min/max size, forcing people to include certain elements like teleporters, 3 tiered bases or so succesfully is what makes it a better challenge.
If you want a competition and you limit something so basic yet so determing it would be like having a timberbuilding competition where people can only use screws no longer than 6 cm or only 1 kind of screw. It severely limits the moods one can set in a map.
For example there is absolutely no way for someone to introduce a real alien feel to a map because all official maps are heavily techno oriented and you can't edit them. If you were for example to impose the rule 'make an alien area in your map which feels and looks so' it would add to uniqueness of a map. Someone could go for having defaced techno textures, maybe editing the original ones to have webs and slime over them, truly ambitious might go and make or look for alien looking images.
We have discussed this however in the other competition and compared to that this one is at least free.