SMT is a good way to extract parallelism when your ISA makes it more difficult to do (with speculative execution/register renaming). ARM, it seems, makes it easier to the point I don't think any ARM CPU has been using multiple threads per core.

I would expect POWER to be more amenable to it, but x86 borrows heavily from the 8085 ISA and was designed at a time the best IPC you could hope to get was 1.