I really don't get why so many are annoyed by this. Lol.
Because TNG set a precedent where not only did the Enterprise have to be the biggest and best starfleet class, but it had to be the flagship too.
The Enterprise-B was an Excelsior because it was the new standard ship class being built, presumably the same as the Enterprise-C.
As I said before..the Enterprise being a little bit of an underdog creates *better* story opportunities, not less.