It has mostly to do with immigration, which even before 9/11 has always been a bit of a hassle.
A few anectodes from personal experience:
I have waited over two hours on a past trip to the states to make through immigration, just standing in the queue at LAX. I'd expect them make sure to have an appropriate amount of staff ready, since they know exactly how many people are incoming on each flight.
On another trip, I stood in queue together with several hundred passengers, while not a single immigration counter was open (the last person there got out his lunch box and left, leaving the usher who was supposed to direct the travellers to open counters standing their completely helpless). No one was moving *at all* for 15 minutes.
I once got through immigration pretty quickly, then had to wait 90 minutes for my colleague who got detained while the immigration officers rifled through his luggage. No explanations whatsoever were given.
And this is just from personal experience. What we read in the news is also not too endearing:
For some reason they now they want more and more information, like social media accounts, and rumor has it if you ever said something slightly negative about certain politicians, you might be denied entry without recourse.
We've had cases were European nationals where held in jail and later deported, because they weren't sufficiently fluid in english to explain what their travel plans where.
While the likelyhood that you run into one of the more extreme situations is very low, it's not the most relaxing start into what is supposed to be a vacation trip.
Once you're in the country, traveling is usually pretty chill. Personally, I never had any negative experiences, but we heard some pretty horrible stories about how people can get treated if their skin color is a little darker. That doesn't make the place more attractive either.
The US used to be one of the dream destination to travel to, but in recent years, more and more people are turned off by the news coming from there. The world is big and there's so many other places to go to. I guess some effort is required to attract these tourists again.