I think the Enterprise-A was decommissioned for a handful of reasons that collided.
First of all was the fact that the Constitution class itself was definitely getting old at that point - the original Enterprise was at least forty years old when it was destroyed, so the class design as a whole was fifty years old, right as the Excelsior class was ramping up production, to the point that they had a variant design already in the works.
Secondly, the Enterprise had just taken some MASSIVE structural damage in the fight against Chang's bird of prey - remember the big honking hole shot through the saucer? That was going to take some significant time/resource sink in order to patch it up and make the Enterprise ready to go on any more long term assignments.
And thirdly, the Enterprise's senior staff were all about to step down, removing a good chunk of the ship's experienced hands, meaning that they wouldn't be there to guide anyone new into the handling of the ship.
Only one of those issues could probably have kept the Enterprise-A flying. Two, maybe. All three? Starfleet weighed their options and decided if they were going to invest in an Enterprise, it might as well be a state of the art ship of the line, not an aging vessel in need of massive repairs and without the crew most familiar with her.