Maybe too many complaints about claustrophobia from the clam shells being closed? People getting stuck because the clam shell got out of it's tracks? I'd think they are easier to clean than a curtain.
When we book a suite they've had curtains for the bathtub. We had Owners Suites a couple times on the Serenade and the Enchantment and they have a separate shower stall with a glass swinging door, and a bathtub.