I wouldn't say it changes very fast. If you look at the language features in the past two releases they're all of the form "allow {existing feature} to do {thing you'd expect to be allowed}", and the library additions are minor.
But it changes often, every six weeks, where e.g. Python releases features only once a year. And it's common practice to install new releases as soon as they come out without waiting for distros to pick them up. That means software starts depending on new features in minor frivolous ways that nevertheless break the build for older Rust versions.
I make an effort to have my libraries support older Rust releases, but I have to go out of my way to check for compatibility. Not everybody does that, and it's unrealistic to expect everyone to—but with a different release schedule that would give fewer issues.