most likely because they don't want to pay Apple a % cut of passes sold via the app. It ranges from 15 to 30% depending on how much volume you are doing via the app. If sold via a mobile website and not the app, they don't have to pay Apple their extortion fees.
Honestly though the friction of having tourists and low-frequency users download an app is probably higher than just visiting a website.
But I really think the checkout flow should remove the need to create an account. Just let single use riders quickly rent bikes with Apple Pay/Google Pay etc