Simply because it is not as easy to hit here as it is to hit the Marriott in Pakistan, and at the same time, it sends a message to the Pakistanis that Al Qaeda is strong and active where they live and co-operating with the US is dangerous.
Tim McVeigh hit us pretty easily. I like to think our intelligence is so good that it won’t happen here but that is mostly wishful thinking. Car bombs aren’t that difficult.