We know the devices you are talking about are true.
We know they are extremely easy to get (wonder if they are on eBay?).
So if the terrorists are real and an imminent threat why haven't they seized the moment? Things like this makes me wonder about the validity of these groups.
Well this is just an opinion but here's my opinion as to why the terror attacks have stopped...
The key component of the word terrorist is "terror".
The terrorists have achieved their goal -- the USA now lives in terror. In fact it's worse than that.
US citizens have lost a raft of "rights" in the name of "the war against terror" (WOT).
Call me a cynic but I can't help but feel that the US government has seen an opportunity to use 9/11 as an excuse to significantly dilute the rights of the individual and introduce a huge range of draconian measures that boost the power of the state over its citizens.
The WOT has done more harm to the USA than WW2 ever did. Key parts of the constitution have effectively been struck-out and the courts allow it because the word "terrorism" is used as a justification.
The USA now thumbs its nose at a range of human rights conventions, especially at places like Guantanamo Bay -- while also (hypocritically) denouncing other nations for their breaches -- even launching war against them to supposedly "protect their citizens" from such rights abuses.
As a result of this, the USA has lost the respect of huge numbers of people who were previously their "friends".
Of course this suits "the powers that be" because, as they make more enemies, the justification for turning this former democracy into a police-state grows stronger.
Now, thanks to a small band of terrorists who have killed fewer US citizens than die annually on that nation's roads, you can't fly on an airplane without being bathed in ionizing radiation or "patted down" by security staff. What's more, anyone who's had to endure the journey through US immigration (at least at LAX) will testify to the fact that there's no "Welcome to the USA" -- it's more like "we presume you're a terrorist and that's the way we're going to treat you". It's not a pleasant experience -- with threats and rudeness completely destroying an last bit of sympathy a traveller may have towards the USA's predicament.
With the USA digging its own hole, there's no further need for terrorists to risk their own lives or waste money actually launching attacks on the USA. There's nothing to gain. The spectre of fear has been cultivated so well by the USA's own government that there's very little a bunch of "insurgents" could do to make things worse.
For a nation that positions itself as the poster-boy for human rights and democracy, the USA has done a wonderful job of dismantling both within its own borders.
I know dozens of Americans and they're all wonderful people -- but their administration -- well that's something completely different. Unfortunately, the importance of patriotism is drummed into Americans right from day one so nobody wants to (publicly) criticise their own country or the administration that governs it. As a result -- the US government continues to do exactly what it wants to do -- which generally means looking after its rich friends (RIAA/MPAA for instance) and riding roughshod over the rights of the average citizen.
But as I say -- this is just one man's opinion. Yours may differ.