(The description below is written within a Trek context.)
The USS Excelsior (NX-2000) was commissioned as a testbed for new technologies by Starfleet Command.
One of these technologies, transwarp drive, was seen as a possible revolution in starship design, with the potential to open up the entire galaxy to exploratory missions. Unfortunately, transwarp physics had progressed well beyond the technology available at the time, and this experiment was shutdown - to some degree of embarrassment on the part of the Starfleet Corps of Engineers.
However, the basic ship design proved highly adaptable, forming the basis of several starship classes equipped with conventional warp drive. The Excelsior herself was recommissioned as NCC-2000, after the retrofitting of standard warp coils to its existing nacelle design.
The Excelsior class spaceframe proved to be remarkably durable, able to withstand considerable punishment both from the elements and Threat vessels alike. This accounts for the considerable number of vessels remaining in Starfleet, even several decades later.