Some people are impatient. Especially at rush hour waiting behind iPhone users fumbling with Apple Pay Face/Touch ID authentication before going through crowded Transport for London (TfL) transit gates that are not that fast to begin with. Unlike native transit cards on Apple Pay like Suica and HOP, Apple Pay EMV Express Transit support is more complicated to enable because it involves not only fare system support on the transit agency end, it also involves banks and bank card hotlist management at the transit gate reader level, which is tricky because reader memory is limited.
I’m sure that TfL and fare system operator Cubic will get there eventually, but even so EMV Express Transit will never be as fast as native MIFARE Oyster cards. Unfortunately it looks like TfL will never bring those to Apple Pay.