The reason why 9/11 had a bigger impact than the attacks in London and Madrid (and they were huge, they should not be diminshed) is because the 9/11 was on US soil. Let me try to explain what I mean.
The USA had not seen war or such a vicious attack from a foreign entity on US soil. It was a first.
The UK had experienced IRA attacks in recent history. Spain had experienced ETA attacks in recent history.
The fact that 9/11 got more attention had to do, in part, with people being shocked that the USA was no longer a safe haven. Not that some people didn't realise that, the government has taken quite a few measures to increase safety but 9/11 made it real, it brought it home.
People in the UK and in Spain didn't need reminding. The scars of terrorist attacks are still fresh. People were used to a thing or two there.
I don't think I can agree with you. The World Trade Centers were bombed in 1993 in an effort to bring them down, but the bombs weren't big enough. Yet it was big enough to kill 6 and injure 1000. It was certainly unsettling, but it didn't stop people from going back to work there. Furthermore, most people have no idea what date it occurred on.
I think 9/11 got more attention because it killed as many as it did and the visual of watching a structure that large go down, not because of the location (that is not to say it being American didn't increase the coverage a bit, but I believe that happening anywhere would have gotten a massive worldwide reaction).
If someone brought down the Eiffel Tower tomorrow it would probably be remembered better than anything in London and Madrid (even if the death toll wasn't as big) because of the visual of that structure (particularly an iconic one) being decimated.