One reason is that overriding "=" would allow you to violate the move semantics of Rust: a = b; is guaranteed to be equivalent to memcpy(&b, &a, sizeof(a)). In C++, you can overload the assignment operator, which lets you do crazy, unexpected things. In the end, it's mostly a tradeoff between convenience (writing a = b; instead of a.assign_from(b);) and explicitness (being able to tell what might and what might not happen under the hood).
It's the difference between the Copy and Clone traits. One thing that Rust still lacks in its standard library is a Clone-like trait for moving data, which is also something that C++ can express, and could be useful in combination with "pinned" object which can't be memcpy'd. There is a crate providing this (via a Transfer trait), but it's nowhere near being a standard.
If this were ever to happen, it would have to be similar to Clone in that it would be explicit, not implicit. This would also make it not as useful as the C++ version, but it means not giving up all of the advantages of our semantics.
