It's because it's an extremely delicate balance (considering reference counting a form of GC, which it is): you have to carefully design the entire language around it in order to get the safe expressiveness that Rust provides. It'd be very hard to retrofit onto an existing design and match Rust's expressiveness.

How does Rust deal with reference cycles?

By giving you enough tools that referencing counting isn't what you reach for. If you do need it, though, we use weak and strong pointers.