I describe it as the Federation taking a “peace dividend” at the end of the Klingon Cold War that disrupted new starship development for a generation or longer. Excelsior-class was the best platform to result from cold-wartime development and thus became the workhorses that lasted through the transition to the more “peacetime developed” heavy explorers like Ambassador and Galaxy-classes.

The real-world parallel are the US Navy Arleigh Burke-class destroyers. They were the peak design at the end of the Cold War, a peace dividend was taken which slowed shipbuilding at the time, their overly ambitious successor classes were ultimately unsuccessful, and it was decided to cancel the newer Zumwalt-class after only 3 ships and instead keep building more third-generation Arleigh Burkes.