It lends itself to a workflow where sometimes, a user checks out a file, and forgets to unlock it. Or goes on vacation. Then you have to forcibly check out the file your coworker locked to get work done. And when he has to merge his changes with yours again, it’s a gigantic hassle.
This exactly describes web development on the Microsoft stack using Visual Source Safe. I don't know if that product exists anymore or still works the same way but the frustration with it is what drove the popularity of Subversion and then later Git.