It's probably a combination of target market and trade offs.
To make SMT-4 perform well you want to have larger caches so that cache contention between the threads doesn't become the bottleneck, but that eats a lot of transistors. It's essentially a brute force trade off between performance and manufacturing cost and IBM is more willing to say "damn the cost" than Intel.
There's also the matter of who needs a machine like that. There is a lot of ugly pointer-chasing code in the world, but to take advantage of SMT-4 it has to be well-threaded ugly pointer-chasing code. You basically need a customer that needs their application to scale and is willing to do the bare minimum necessary to make that possible, but not spend a lot of resources actually optimizing the code once they get it to the point that throwing more hardware at it is a viable alternative. That's the enterprise market in a nutshell right there, and that's where IBM lives.
To make SMT-4 perform well you want to have larger caches so that cache contention between the threads doesn't become the bottleneck, but that eats a lot of transistors. It's essentially a brute force trade off between performance and manufacturing cost and IBM is more willing to say "damn the cost" than Intel.
There's also the matter of who needs a machine like that. There is a lot of ugly pointer-chasing code in the world, but to take advantage of SMT-4 it has to be well-threaded ugly pointer-chasing code. You basically need a customer that needs their application to scale and is willing to do the bare minimum necessary to make that possible, but not spend a lot of resources actually optimizing the code once they get it to the point that throwing more hardware at it is a viable alternative. That's the enterprise market in a nutshell right there, and that's where IBM lives.