Well, not anybody likes to release their meshes. The reasons are variable...
Someones see their meshes as a son, and think that not everyone is skilled enough to play with them. It´s indeed a fact.

Others had bad times when released their meshes. Eric Peterson, from the old Wolfpack, got in troubles with the law cause an ass***e solved to sell one of his meshes and made Paramount to act.
And others got irritated cause the horrible kitbashes made with their meshes, even when it was not allowed. There are peoples that got frustated when a realese is made, but no renderings are made or posted. For the last, the ones that try to pass a mesh built by other as if it was built by them...
Well, when you publish something on the net, is hard to keep control. I think it is what annoys more a lot of peoples. Others simply don´t mind at all. Do what you want. And so on...
Private releases are still a good way to have some rare meshes, and I´m lucky enough to have a few.

From my side, until today, I only released one mesh built by myself and 03 conversions, but if I had finished others, for sure I would release them. Sadly, freetime isn´t my friend anymore.
Just be gratefull for what the others released (or not released, but showed). Peoples have different minds, fears, and desires, what makes them do this or that.

Release a mesh is, at the end, a personal decision.
