The prototype Excelsior spaceframe was a testbed for an experimental warp drive system (often called transwarp as it went beyond established speed records) which was developed over the 2270's and 80's. While the theory of the new warp design was sound, it proved too complex for the manufacturing and computing capabilities of the era. The resulting drive was extremely complex and prone to develop failure points. Adding to the problem, common impurities in dilithium matrices which had little impact in standard warp systems affected its performance significantly. This meant that even when working properly it couldn't deliver the higher speeds it promised.
Eventually a more conventional warp drive system was installed on the prototype spaceframe instead, which was commissioned as NCC-2000. The success of the design in all other areas led to other spaceframes of the class being ordered for construction almost immediately, and before the 23rd century was over Excelsior-class vessels were a common sight across the Federation, replacing the aging Constitution-class heavy cruiser as a multirole explorer.