It's just that some folk don't like change kent.
Ok the old style tubbed adhesive has worked for donkeys years, but things have changed.
Porcelain and large format tiles have become the norm, you cannot used tubbed for these.
Showers have become way more powerful, so more water sloshing around means potentially more water absorbing through the grout, no cementious grout is waterproof, it' waterproof. so any little weeps of water on tubbed adhesive will wreck the job. Cementitious is more water resiliant.
People want tanked showers and wet rooms, you cannot fix tiles to these as the tanked substrate won't absorb the adhesive.
So there are some major factors here for not doing any showers in tubbed.
I wouldn't rule out tubbed adhesives, I still use some tubbed on small ceramic tiles as it suits the job nicely, but only in dry areas, never showers.