because each core has its own instruction counter

Both

> Because you can have a core set up for each branch

and

> because each core has its own instruction counter

You can have each core be a tighter loop because you can limit the types of cases it handles.

But you also have your own instruction program counter you can take all sorts of branches in a way that you can't with SIMD, because in SIMD as the name implies you get only a Single-Instruction to deal with Multiple-Data. So if you need to change the behaviour depending on the exact value of the data, you are better off using separate cores than wider vectors.