First of all: Rust used to have classes. They were removed because other concepts were seen as more fitting. That happens to be the case for many aspects of the language: Rust also had a GC.
The issue from my perspective is: if you add classes, Rust would have classes and traits. It wouldn't solve much. And separation of implementation and data has some really neat advantages: for example, passing data over FFI boundaries becomes far more natural, in both ways. As long as you know the data layout, you can use this data in Rust like any other. (Just implement traits on top of it)
I do a lot of FFI work and am surprised how easy it is to wrap foreign code into a Rusty interface.
It's a little bit more fragmented, but in a good way for my taste.
Finally, the OO-nerds perspective: there are about a hundred opinions and implementations of what proper OO is (from CLOS over Java/C++ to Ruby and Smalltalk message passing), Rust adding number 101 isn't much of a problem.