The other ships weren't comparable to taking on the Enterprise (if needed)? Not fast enough- not powerful enough?
Maybe the ships were in various stages of refit-repair-crew shore leave?
And I always assumes space defence of the Sol System was patrolled by low warp, heavy powered ships that weren't meant for really leaving the parameter of our system. Plus Earth itself has that giant beast of StarBase that I personaly assumed is armed to the freakin' teeth.
Then there's the point that space is HUGE!!! You may have a fleet of 10000 ships and it means shit in the vastness of space. I posted a thread once with the size of the Federation and the sheer volume of space that takes up. It's insane- even with warp drive.
And then that leads into the threat factor. I never really thought threats of a system were as large as some series have suggested. And that's primarily because of the size of space. And how defendable the major systems would be. Major enemy encounters probably primarily involved skirmishes. Some lesser planets get raided, or even occupied by an agressor. Starfleet takes it back, and life goes on. No giant everyday planet killing silly plots...
One other thing about sending out someone to pursue the Enterprise- I think there would be some political interference as well. The Excelsior failed immediately, and there may have been intensional delays to react because some people didn't want to stop Kirk.