>>357272093
The reason is that the tech demos and pre-release publicity was really good. They showed off the physics in sandbox environments with some rough assets and it looks great. The general reception was that "These gameplay things would be great in a game!"
Then it came out and what they showed off wasn't just some tech demo, it was actually the game. 
Consider playing the multiplay mod, and generally modding it in general. The game itself is meh but the sandbox potential is still good.