[steveklabnik]

I think at the time Cargo was made it was just far simpler to implement. It's not just that, it's also rustc version, sometimes environment variables... much less likely to cause problems by keeping it per project. Of course that stuff still needs to be kept track of, but like, "to get a clean build, kill this directory" seems easier. Not sure if there is an explicit justification written down anywhere from 11 years ago.