Because rustfmt does not have enough proper customisations.

I just started Rust a few days ago and after my code became >20 lines (following the docs, understanding the basics), I decided to look for a formatting tool for Rust.
I found out rustfmt, read :

Rustfmt is designed to be very configurable. You can create a TOML file called rustfmt.toml or .rustfmt.toml

and was kinda happy.

And since in cmake I use BreakBeforeBraces: Allman, I looked for a similar option.
Found out in the docs, it was not available in stable. inner-thoughts: How old is Rust? I already waited 4+ years before picking it up. Well, let's look up the unstable features on GitHub.
There are 2 configuration options relevant to the functionality.
Both of them have enough parts where they don't work, so they are useless even if I were to use the unstable version of rustfmt.
And it seems like either none of the people participating in that issue is good enough to make changes in its source code or the rustfmt team doesn't want that feature implemented.

And if someone requires a reason for Allman style for BreakBeforeBraces:
The Allman style makes it much easier to identify block scopes without having to rely upon IDE features such as highlighting.