That's really just restating 1) organizing such an attack is harder than it looks. Part of organizing such an attack is finding homicidal maniacs willing to die for your cause.
It's also dealing with the repercussions. 9/11 was different because they essentially just came into the country to carry out the attack. I think it would be hard to get a Beslan type attack here because they would first have to have a base of support within a community in order to plan and support that type of attack. Once the attack was carried out - that community would be subject to repercussions. It's not like there's a Muslim country just over the border anywhere in the US that they could run to. If it was found out that say some mosque was the center of operations - it's probably an even bet that that mosque and the community it supports would be subject to all sorts of nasty attacks.
It doesn't happen very often - but people still get tied behind pickup trucks and dragged til they're dead and their body parts are flying off here in the US - doing a Beslan and killing a couple hundred school kids would make people go apeshit.
If you're a Muslim in the US - and have been here for say 15 years - you've got to know a good part of the country would use a school attack to start going after you. And it's also well known worldwide that the US is full of guns - and liberals constantly tout the US as the murder leader of the world - so the "other side" HAS to know there would be repercussions to any sort of school type attack.