I think the distinction is that Linux is very different from Windows in a lot of important ways, while Git and Mercurial are relatively similar in what they do and how they work. If someone invented a VCS that completely revolutionized the process (I really don't know how it could, but just hypothetically), then it could serve as true competition to Git.
This is one reason why D has failed to really take off. Both D and Rust are looking to essentially take C++'s place. D is C++ improved, while Rust changes the whole model of managing memory and also introduces some new paradigms (more type inference, pattern matching, etc.). D is clearly better than C++, but the two are just similar enough to make C++ a more preferable choice due to its immense popularity compared to D. Rust changes just enough to give it a chance at being a serious contender: the value may be higher than the cost of leaving C++ for many people.
The same applies to Git (C++) and Mercurial (D). Perhaps even more so, because while Git isn't perfect it generally "just works" for almost all use cases; Mercurial is just some simplicity and elegance on top of some workflows (which are sometimes emulated with wrappers over Git, like Legit). C++ on the other hand has a lot of warts.
This is one reason why D has failed to really take off. Both D and Rust are looking to essentially take C++'s place. D is C++ improved, while Rust changes the whole model of managing memory and also introduces some new paradigms (more type inference, pattern matching, etc.). D is clearly better than C++, but the two are just similar enough to make C++ a more preferable choice due to its immense popularity compared to D. Rust changes just enough to give it a chance at being a serious contender: the value may be higher than the cost of leaving C++ for many people.
The same applies to Git (C++) and Mercurial (D). Perhaps even more so, because while Git isn't perfect it generally "just works" for almost all use cases; Mercurial is just some simplicity and elegance on top of some workflows (which are sometimes emulated with wrappers over Git, like Legit). C++ on the other hand has a lot of warts.