Many cities have their own cards, that are used for transit only (so no vending machines etc., and no interoperability between cities), increasingly many places just let you use your credit card in place of the special transit card, some places (Germany, some parts of Scandinavia) don’t have ticket gates at all.
They now made the decision to completely retire the current RFID solution, because the chip vendor discontinued the lineup it relies on. Its replacement proprietary card is based on NFC tap-to-pay, so on top of terrible product compatibility during the transition period we're also getting a nice extra tap-in delay out of it.
I believe the reason is mostly historical (it wasn't deemed necessary when most of the system was built, and the later attempt to introduce the gates later failed for a bunch of different reasons)
The challenge in the UK is getting tap-to-pay through bank cards onto all the little local bus operators.