If I'm not mistaken, the Excelsior miniature was a much cheaper piece of craftsmanship, kind of a quickie, and would not have looked very good in a starring role, whereas the 8-foot TMP Enterprise was a high-end beauty model. So they saved a ton of money and
got the better-looking ship by re-using the 1701 job.
What I regret about it is, they thought this "-A" thing, a shortcut by the art department, was a precedent to be carried on forever. I wish that when TNG came about, they had given the Galaxy class Enterprise its own original hull number instead of a D.
The Enterprise B and C should have had their own original names altogether, for my money. The stories would have been just as good, and less wrapped up in an unrealistic reverence for Kirk's ship and its number.