I think looking at the ships' different roles may answer that question. The Miranda-class is a backline support and science platform, only called to the front during extreme circumstances (Wolf-359, Dominion War). They can be efficient at simple taskings like specific missions or supply runs. Like its successor, the California-class, they're also the mass produced, easy to build/repair workhorse type, so a simple, tested design just works over a longer life time.

Whereas the Constitution-class is a much larger, more complex, and line serving Heavy Cruiser. Presumably being on the frontline of exploration and military intervention, they have a higher need for the bleeding edge tech The Federation can field, especially with the fleet flagship USS Enterprise. Its unique hardware would also make it harder to repair and maintain which probably affected the lifespan of the class as a whole.

Like others said, it's most likely more sensible to do a full redesign whenever a new frontline or special class is called for instead of relying on retrofitting to keep up with enemy tech advances. Especially with core components like warp core upgrades, computing types (ie Galaxy-class moving to isolinear), or completely new tech like the Intrepid-class' bio-neural circuitry. That could maybe answer the shorter lifespan only lasting 1-2 retrofits before class charge (ie NCC-1701-A/B to the NCC-1701-C to the NCC-1701-D/E)