Because more people play on trade server then servers with custom maps.
That actualy depends more on the quality of the gameplay. With maps that have a poor gameplay it indeed is true, but with good gameplay (which with enough playtests can be achieved) the chance that people play it in pub is much larger.
There are still many servers that specificly want custom maps even if they are a bit less quality in detail. As long as the gameplay is good they want it.
I even noticed that with MvM. There is a small community (since mvm is quickplay based most probably never will witness such map) yet that community does still play the early day custom maps mixed in with more recent ones.
Dont make the fear of people never playing your map true. The chance is allways there, but to me with trade maps i still get that feeling that people just pick a random one rather than picking the best. Since its ment for trading they dont care about gameplay or even if the map looks good and already accept a map like achievement_idle. A good looking map does increase chances but by far not enough.
Especialy since gamebanana has alot of spam of such maps i would actualy say that as trade map you have a far smaller chance that people will see it. It might be on more servers overall, if its only on for 10 minutes a day its often not worth it.
While if you have a quality map, that map can be in a normal server and be played much more.
How do you make rocks without it lookin like jello in slow-mo
This is probably the best guide made to explain displacements rocks:
http://forums.tf2maps.net/showthread.php?t=798
not everyone might have the same end result as style (some prefer sharper rock edges for example - and some do it based on other system) the method is simply the easiest to learn and use.