Jemalloc added over a megabyte to every project for only questionable gains, and it was awkward and unwieldy to remove it. While there are good reasons to use a different allocator depending on the project, Rust defaulting to this type of behavior failed a certain personal litmus test on what it wanted to be as a language in that it felt like it was fighting the system rather than integrating with it.

It also does not give a good first impression at all to newcomers to see their hello world project built in release mode take up almost 2MiB of space. Today it's a much more (subjectively) tolerable 136kiB on Windows (considering that Rust std is statically linked).