[1] https://github.com/rust-lang/rfcs/blob/master/text/0066-bett...
E.g., if someone thinks "I'm going to target 2015 because I want my code to run on the rustc shipped with various slow-moving Linux distros", it doesn't help, because you might still not be able to target their code, unless they specifically target an older version of rustc, which nobody does.
There has been discussion of a Rust LTS channel alongside stable/beta/nightly, which would try to solve that problem, but it has not been prioritized yet: https://github.com/rust-lang/rfcs/pull/2483
An actual frozen language is also a possibility, but probably won't happen until more work happens on an independent specification. Which, in fact, people are also working on: https://ferrous-systems.com/blog/sealed-rust-the-pitch/
Rust dearly needs a stable specification, it is the main blocker why the language hasn't been more widely adopted.