They're fast, have lots of memory and IO bandwidth and can do some cool other tricks (I can't remember the name right now, but they have thing for PCIe devices to participate in cache coherency, their in-system protocols scale better to more CPUs, ...)
The main reason is the partnership with nvidia. You get nvlink to the CPU, which you don't get on Intel/AMD (I believe). Other than that, I don't think there is a real competitive reason. The support and future timeline from IBM is lacking. I'm honestly surprised they are still big friends.
