The Sovereign ClassEdit
Described as a "Super Intrepid", the Sovereign class was designed to replace Galaxy class starships on patrol , freeing those ships for exploration, while providing similar crisis response capacity.
The design had a lot in common with the Intrepid class starships, borrowing design elements liberally.
The Sovereign class was a successful class of ship and became common in the Federation in the late 24th and early 25th centuries. Although intended for patrol and crises response, the Sovereigns were also not shabby at exploration or science investigation, and when possible were used for these tasks as well.
Many successful Starfleet Officers served on a Sovereign at some point in their careers.
The Soveriegn class appears in 1996 in "Star Trek First Contact"
The Enterprise-D was designed by Andrew Probert. His original painting of a "Future Enterprise" was a lark. Gene Roddenberry saw it and asked to use it as the design for the Enterprise-D in Star Trek The Next Generation.
Probert, being wise, liscenced the design to Paramount retaining the rights to the thing himself.
That meant that every time a Galaxy Class Starship appeared on screen, Paramount cut Andrew Probert a check.
During Production of Star Trek Generations (1994) it was discovered that the standing sets for the Enterprise-D, some of which dated back to Star Trek Phase-II (1978) and Star Trek The Motion Picture (1979) - all these sets were nearly unusable for modern film production. This is why, in Generations, the Enterprise-D looks dark. They simply couldn't light it properly.
So the decision was made to destroy the Enterprise-D, and replace it with a new ship.
At the same time work was beginning on the new series, Star Trek Voyager. New sets were built which were intended to be multi-use Corridors, engineering and sickbay sets that were used for the USS Voyager (And the USS Equinox) were also the corridors, sickbay and etc used for the new Enterprise.
A new exterior design was created. It was the same designer who worked on Voyager an all the Star Trek series of that time, so the look was consistent. This is why the Enterprise-E comes off looking like a Super-Intrepid. Because the two ships share a lot of sets and design philosophy.
But this left me to explain why Starfleet built a whole new ship to replace the Enterprise-D.
I created the above explanation for the E-D and the E-E for ST-OM. you should not consider it binding. Interestingly, in ST-OM, the Enterprise -D was not destroyed. The ship that becomes the Enteprrise-E in Canon, in ST-OM becomes the new USS Yamato.