Because Microsoft didn't make it?
Shame on me for not jumping on the bandwagon and instead actually appreciating a companies product for what it is.
Gmod is a one of a kind game and it has a wide user base. By all definitions it is successful. But the nature of the program (relying mostly on user generated content) makes it tedious to use and difficult (if not impossible) to optimize.
For those with an old computer or lacking a decent internet connection, Gmod can be impossible to play. Joining a server is often a lesson in frustration as you end up having to download every little piece of crap they have decided to cram into it. At least with Blockland the number and size of files is typically smaller and more manageable and the game doesn't require even decent hardware to run it.