It isn't. I don't intend to generalize to everyone using Rust. But there is a large overlap between Rust evangelists and anti-GPL activists, it seems. I don't want to believe this, but it almost looks like a corporate conspiracy to undermine free software.
AFAIK there definitely is a preference for Apache/MIT dual licence but this is probably because of the influence of the Rust compiler itself. Most of the initial Rust code was written by people involved with and in service of the Rust compiler. Since that’s licensed MIT/Apache, their code had to be as well if it wanted to be used in the compiler.
There were people who used funky licenses like WTFPL (whatever the fuck you want to) and Unlicense (public domain) but they came around and licensed to MIT/Apache for uniformity with the rest of the ecosystem.
Maybe if the compiler had been GPL the ecosystem might have been as well, but that’s a what if. We’ll never know if a hypothetical GPL Rust would have had the same path to success as the existing Rust.
