I don't think that Americans are becoming less patriotic, but there is a saying that time heals all wounds. The further that we get away from 9/11 the more people will forget the significant impact that it had on everyone. Especially now when people are grown who are too young to remember.
You really asked two different questions there. I don't think Americans are being less patriotic but I do think we are forgetting more and more about 9/11 each day and year. Big events in history slowly fade over time as they are replaced by bigger events or more pressing events that get our attention.
The low number of visitors (250,000 annually) as opposed to predicted visitors (2 million annually) to the Flight 77 Pentagon Memorial does not mean that Americans are becoming less patriotic and forgetting about 9/11. I think that this happens in all tragedies and people just want to move on with their lives. They will never forget but they can not stop living because of it.
No, average attendance to the Flight 77 Pentagon Memorial of only 250,000 does not mean that Americans are becoming less patriotic and forgetting about 9/11, because 250,000 is still a large number of visitors. When most people think of a 9/11 memorial, they want to go visit the memorial in NYC. The one in NYC is also easier to get to. That might explain the low numbers.
Just because the predicted 2 million aren't visiting the memorial; it doesn't mean that those 2 million people aren't remembering the victims of Flight 77. There are more ways of remembering the victims other than visiting a memorial. As the above said, financial issues are a problem. Many people throughout America will not be able to afford to go to the memorial, but they may remember them in another way.
I do not believe the low number of visitors to the Flight 77 Pentagon Memorial means that Americans are becoming less patriotic, nor are we forgetting about 9/11. I believe the major problem with the memorial is the fact that its completely inaccessible to the majority of Americans who can no longer afford to travel long distances due to financial pressures. I would love to visit the memorial, however I can't afford to take a week off work and pay the expenses of traveling while maintaining the plethora of bills that I have to pay.