How many indie Source games are there? It's not only more difficult to get a Source engine license, but it's also a lot less appealing. Think about it:
Monthly Updates(likely faster for licensees)
Advanced tools, with distributed compiling, realtime previews, visual material and cinematic editor.
UnrealLightmass, which is static like Source, but can handle more advanced(like volumetric) lighting and more lights overall.
Dedicated licensee wiki
How many AAA games use this engine now? Oh, here is a list(not all of them are AAA): http://en.wikipedia.org/wiki/List_of_Un ... l_Engine_3
Scales back well on old hardware
Outdated tools, even if they were updated recently, they have no modern features like the UDK and CryEditor have.
Why go through the trouble of using licensing Source, when UDK is easier to and when the UDK has more advantages. I could see, if you nearly completed your mod, you wouldn't want to have to redo so much. But think about it, for example, Ivan's secrets; how many maps have they even done? Maps are really the only thing you can't port. Pretty much everything else is easy to port to another engine as long as you got the source files.