I think that's because surprisingly, it's still a marketable/profitable game. If only it weren't, then maybe it would eventually be open sourced. Only flaw I find is that due to it's age, it wasn't really designed with maximum concurrency in mind, and doesn't utilize the full capabilities of modern hardware, so the simulation slows to a crawl after a certain size.
The game actually does support decent threading, but it relies on old RDTSC behavior and will crash on modern CPUs if you allow all the threading to run. I nagged EA a lot to Open Source SC4 or allow fans to fix bugs on Windows version, but they are stubborn :(
The Mac version is published by Aspyr that has rights to the source and to release updates. For example the Mac version is 64bit now. Windows version is 32bit thus heavily limited regarding mods.
