As for the problem at hand. Earth falls under the Vulcan sphere of infuence and the Klingons do not wish to incur their displeasure.
The Vulcans don't seem likely to inspire fear in the turtleheads.
ENT had the chance to establish why Earth had been ignored by more advanced empires for so long; the Trekverse has always been unrealistically crowded with inhabited worlds, so maybe that's the explanation. Earth wasn't out of the way, but there was a lot of stuff inbetween that the Klingons found more interesting or put up more of a fight. The Andorians would have been no pushover. The only reasonable explanation for Earth remaining free until it was ready to explore the stars is sheer dumb luck. Maybe the Vulcans ran interference, but that alone wouldn't have kept all the baddies at bay.
ENT should have been about this danger from the first. Earth had simply been lucky all those millenia; now it was time to wake up and find out what was out there, before we learned the hard way. There was never any reason to make up any new enemies. The old ones would have been perfectly good.