This answer contains spoilers, so if you haven’t seen the film, look away!
This is all speculation. Arranged roughly in order of plausibility.
The events of the 2009 film
In the 2009 film, Nero has a ship with “advanced and formidable weaponry” that destroys the USS Kelvin, at least forty-seven Klingon war-birds, and reduces six flagship Federation ships to shrapnel. It could have done the same to the Enterprise.
(Sidebar: this raises the question of why nobody on Earth or Vulcan, spotted Nero’s ship, and the drill in their atmosphere. I’m not talking about automatic defences — just looking with their eyes. We know Spock’s mother saw it, and apparently she did little about it.)
The Vengeance drops out of warp fairly close Earth, and it’s larger than any existing Starfleet vessel. It also seems to have followed the Enterprise closely in warp (“advanced warp capabilities”). It points large and scary guns at the Enterprise. Sound familiar?
I don’t Starfleet would pitch a large number of ships and lives at something that outmatches and outguns them, given what happened last time. Of course, this is the very eventuality that Marcus was trying to mitigate against when he built the Vengeance.
The attack on Starfleet HQ
Marcus gathered all of the Starfleet officers with “ships in the area” in the conference room before Khan’s attack. There are two possibilities for each officer:
They're killed, and their ship needs a new captain or first officer. I don’t know how quickly Starfleet could promote somebody to captain one of their flagships, but I’d guess it’s more than a day. And given the size of the Vengeance, I don’t think they’d send anything but a flagship.
Kirk is an exception. He’d already captained the Enterprise, so he knew the drills associated with the ship. Experience is less of an issue, and Marcus is willing to use him as a pawn against Khan and the Klingons.
Or, they’re injured. When the crew are boarding the Enterprise, Bones refers to the attack as very recent. (I think he says ten hours, but I can’t remember.) The other events occur within the space of a day or so, so they’d be sending a captain who was injured into the thick of a battle with about a day of recovery. Considering that Kirk was one of the least injured people in that room, and he wasn’t fit for duty, I don’t see that working well for them.
They just don’t know
All the shots fired on the Enterprise by Marcus occur in subspace (while at warp). Their scientists believe that using weapons while at warp is impossible, so they might not realise that the Vengeance has fired upon the Enterprise. Further, since Scotty powers down the weapons almost immediately, Earth scanners might not register the ship as hostile. (Heavily armed, yes. Firing on the Enterprise, no.)
Starfleet may even think that the large ship is assisting the Enterprise. The Enterprise came out of warp with serious damage (but that damage couldn’t have happened at warp, so how did they warp to near-Earth vicinity?) and is unable to communicate with Starfleet. Given their proximity, this is a possibility.
And when Khan starts firing on the Enterprise, and the torpedoes explode, it’s all over within a few minutes. If they couldn’t get a few jumpships to defend their headquarters in that time, assisting in a starship battle is probably a bit much for them.
Edit: This gets shot out of the water when you consider that the Enterprise is able to communicate with New Vulcan and the other Spock, but not with Earth, which is practically within sight. We need to ask why Spock does not simply tell Earth “By the way, that big black unmarked ship? Yeah, that’s trying to kill us.”
Marcus is on the Vengeance
During this battle, the Admiral of Starfleet is involved. He may have the veto power to say, “Don’t come up and join the fight!”. Earth command may also be scrambling to get hold of him (since they probably think he’s still on Earth) to get authorisation to join the fight.
A possibility that I briefly considered, then dismissed, was that Marcus told Starfleet that Kirk and the Enterprise had betrayed them, and needed to be destroyed. I find this unlikely because I think sufficiently many Starfleet officers would want them to stand trial on Earth for these supposed crimes, not just be executed by Marcus.
Sabotage by Marcus
We know that Marcus has nix-ed the Enterprise’s warp core. If he wanted to, he could probably do similar damage to other Federation ships. And it doesn’t need to be serious damage: enough to keep them out of the fight for a few hours is all he needs. (This one is perhaps stretching it a bit thin.)
Those are the main reasons I can think of. Most of them are flimsy, and I could probably poke holes and counter-holes in it for a while, but I think it conveys the major ideas.
None of them are discussed or mentioned explicitly in the film, so if somebody has reasons based on the events of the film, then I’d prefer to read that as an answer.