GPL is basically a contract that code will always be shared if distributed. This licence ensure any distributed library will give your code (legal standpoint).

MIT is more a use as you want licence. Devs can take the code, build and distribute binaries without restrictions.

They are not made for the same reasons. If you want to write Libre software, GPL is your friend.

As other says, rust build statically by default, having a MIT ecosystem help to spread the langage. Yet, this also mean someone can see its code used in a commercial product without sharing back, credits or counterpart.

Different uses.

For those who dunno:

Ferrocene is the main Rust compiler - rustc - but quality managed and qualified for use in automotive and industrial environments (currently by ISO 26262 and IEC 61508) by Ferrous Systems.