Because of what happens to plastic in the first week.
Plastic scratches. Every scrub leaves microscopic grooves, and those grooves hold a thin film of bacteria between cleanings. It's the slick coating you feel inside a plastic fountain after 2 or 3 days. You can try washing it off. It comes back.
That film carries a taste and a smell, and a cat's nose is far sharper than ours. The pattern is always the same: Cats love it for a few days, then they drink less, then they go back to the tap.
Ceramic avoids the scratching but chips, and a chipped glaze gives the film somewhere to sit again.
304 stainless steel is non-porous. No grooves, nothing to cling to, and the whole thing goes in the dishwasher. Same material as a surgical tray, for the same reason.