If you want (what I think) is the real reason: TPTB probably felt that since the Constitution Class Enterprise was a "hero" ship, there can't be another ship on screen that looked like it, since it could cause some confusion among the audience.
But that's just my two cents worth.
That is true! I remember reading something to that effect in star trek communicater! But I like to think the Trek universe reason was because the Connie had too many weak points( i.e. the thin neck and pylons)!