Institutions run every kind of promotion. A cashback offer for switching a chequing account, a waived monthly fee for the first year, a bonus rate on new deposits, points for hitting a spend target. Deciding to run one is rarely what holds a promotion up.
What holds it up is everything after that. Each kind of offer gets handled its own way. A rate change goes into the core, a fee waiver becomes a flag somewhere, a cashback is a credit someone keys in by hand, a bonus lives in a spreadsheet. Nothing ties them together, so there is no shared idea of who qualifies, how a customer enrols, how two offers interact, or when the offer is meant to stop. When a promotion should end, someone has to remember to end it, and when they forget, the offer keeps running for months past its end date.
The billing layer has no single concept of a promotion, so every offer is a one-off workaround, and workarounds do not expire on their own.