It is hard and likely expensive to require every single reader in every single city to be networked:
> Because Clipper operates in multiple geographical areas with sporadic or non-existent internet access, the fare collection and verification technology needs to operate without any networking. To accomplish this, the Clipper card memory keeps track of balance on the card, fares paid, and trip history.
It is hard and likely expensive to require every single reader in every single city to be networked:
> Because Clipper operates in multiple geographical areas with sporadic or non-existent internet access, the fare collection and verification technology needs to operate without any networking. To accomplish this, the Clipper card memory keeps track of balance on the card, fares paid, and trip history.
https://en.wikipedia.org/wiki/Clipper_card#Technology