If most of an ecosystem chooses a specific license (dual licensed in Rust's case), the simplest thing to do is choose the same license as everyone else.
Regardless of what others do, the best thing to do is to choose the best license for one’s own software. One which preserves the freedom of one’s users and the openness of one’s code.
Sadly people don’t always do what’s best. We sometimes do what other people are doing on the theory that maybe someone else has thought it through and already decided that it _is_ the best thing to do. It’s not perfect, but then heuristics rarely are. But it’s cheap to implement.
