The dislike started earlier than the rise of pervasive threading. As another poster said, every read turns into a write, meaning, everytime you pass a reference into a function you have to increment the counter, and decrement when the function exits. It turns out, the vast majority of of inc/dec ops are pointless ones like this, which is considerable overhead for literally no benefit.
You can elide some of them using a sort of borrow analysis. But borrowed refs can only be used in certain ways, and so this analysis isn't perfect and can't eliminate all of the unnecessary overhead. This was all hashed out in the 80s and 90s IIRC, before multithreading even became a big issue.
The complex analyses needed to make RC perform well end up destroying the simplicity argument, and even after they still aren't competitive with tracing.
You can elide some of them using a sort of borrow analysis. But borrowed refs can only be used in certain ways, and so this analysis isn't perfect and can't eliminate all of the unnecessary overhead. This was all hashed out in the 80s and 90s IIRC, before multithreading even became a big issue.
The complex analyses needed to make RC perform well end up destroying the simplicity argument, and even after they still aren't competitive with tracing.