By "sell," I assume you mean "get people to use." (Few will pay for software that they can legally download, unless they are somehow convinced to donate some other way. Few will pay for support for software, especially if it is good, which means that it is intuitive and reliable enough to not require support.)

Some (definitely not all) open source software lacks the other 90% of the stuff people want, including good design (visual or otherwise), good documentation, etc. Many new features that commercial software has are important (it's not obvious because different users depend on different sets of those new features).

The exceptions exist (for example, Firefox is well-designed, and has lots of new features), but they are just that - exceptions. Try to find a good 3D modeling package that's open source. Blender you say? Not many in the gaming industry uses, and that's because it simply isn't competitive with 3ds Max.

Also, some people (myself included) have moral problems with the GPL, so they try to avoid using GPLed software whenever possible. (Things are getting easier for me because most of the open source software I use today is BSD or MIT licensed.)