The Old Mixer wrote:
Another idea...we don't know that all of the Constitutions were doing the exact same type of mission as the Enterprise at the time of TOS.
We know from the Star Trek Writer's Guide that the Enterprise is the biggest and most powerful ship that Star Fleet has, one of our capital ships as Christopher points out.
And we know there are only twelve like it in our possession.
So I think there is no way Star Fleet could afford
to send them all so far off into distant unexplored space, for extended, isolated, and autonomous missions.
Roddenberry wanted to put Kirk in situations where he had no boss looking over his shoulder, where he would have absolute authority to do big, dramatic things. The specific Horatio Hornblower novel that inspired this idea had to be Beat to Quarters
(known as The Happy Return
in U.K. editions).