You are mistaken about revenue collection systems for public transport (and many other technologies or indeed public services themselves). The Oyster card was one of the very first of these electronic pays-as-you-go systems (after Hong Kong's Octopus) and might have required significant R&D (I don't know) but most middle-size cities buy the technology (or key parts of it) off-the-shelf not from other cities but from large companies who specialise in serving the governmental sector (including my employer).
Each local transport authority or agency will have many specific requirements about everything from accessibility and fare structure to means of payment based on local geography and sociology, politics and even rules defined decades ago. That's why it's a little more complicated than just installing consumer software but it's certainly not like rolling out a completely new solution with bespoke technology every time. The relevant transport authority might also contract out the maintenance or back-office (things like a call centre to handle complaints), possibly to the same company.
Cities and local authorities also routinely receive visits from local politicians and experts facing similar problems at the other end of the world (e.g. going to Hong Kong to see how the Octopus system works on the ground) and are typically quite open about innovations like this one (certainly compared to your average technology company) so there is a lot of knowledge sharing going on at this level. But since the technology often belongs to the contractor, the city or government typically isn't allowed to share it.
Now, if your question is why there is a distinct brand in each city and no nationwide inter-operability, that's an entirely different question. I know only one system offering this (the Dutch OV-chipkaart system, which was developed to replace an older nationwide paper ticket system) but it is very very complicated to pull this off, not from a technology point-of-view but from a policy and funding point-of-view.