The problem with Zytiga is that it's a first-gen ARSI that requires taking a steroid with it, and steroids themselves can cause or exacerbate side-effects for many people.

The second-gen ARSIs ("lutamides") do not require a steroid, have fewer side-effects for many people, and are somewhat more effective, but they're also *very* expensive (especially in the U.S.), so insurers probably try to steer doctors away from them.