Starship Troopers

Or.. hear me out on this... the belters did it using stealth Martian technology!

On earth, there's just too many whistleblowers. Like 9/11 there would be at least thousands of insiders.
http://www.thebestpageintheuniverse.net/c.cgi?u=911_morons

Plus the fact that we have defenses which would spot slow moving rocks from very far away.

Now the Belters don't have the beautoracy problem, they hate Earth, etc. I'm not sure where they would get enough money to buy the not-so-top-secret stuff from Mars, but if you rule out everything impossible, then Occam's Razor points to this.

I find a 911 conspiracy more reasonable than a Chupacabra, all a person has to do is reign back the 'scope of the 911 conspiracy for it to be reasonable, but even with that I don't believe the American government caused 911.

I'd imagine the humans noticed the asteroids before they hit, they just must not have had Bruce Willis around to blow it up Armageddon (1998).

But sure, could go with the belter theory as well.

Pretty sure it was an asteroid from within the solar system nudged by a stealthy scout ship of theirs, your suggestion of "just attack the planet with their fleet directly then if they are that close anyway" not being possible due to the human defenses in system I assume.