This is a great snapshot of US men's marathon decline. I cannot believe how many people on this list are from the 70's and 80's.
This list is only from Boston. I think if you look at a list of all-time best US times (that includes *every other marathon*) you'll see lots of improvement from the 70s and 80s.
But you don't. Just looked at the all-time list. The number of US born/US raised who've run faster than Salazar/Beardsley ran 2:08:50-someting at Boston in '82 is five.
While there is more competition today it is important to remember that it has been nearly 60 years since an American has held the Marathon world record with any regularity. Khalid Khannouchi is the one outlier in 2005 but Ethiopians and Kenyans were in the mix form the late 50s onward and the rest of Africa since the 1980s.
What has changed is how far off the world record the top Americans are and how they no longer seem capable of winning major marathons as they once did. Take the duel in the sun in 1982 not only did Salazar and Beardsley finish 1 and 2 at Boston but they ran only 35 seconds off the world record pace. This wasn't an outlier as Salazar had fast times and victories at other major marathons and Beardsley had won London a year earlier and also Grandma's where he ran 36 seconds off the then world record.
So while increased competition may be what has kept American's off the podium in the major marathons it doesn't explain why they have dropped so far behind the world best times.
As others have suggested I think distance running is just not as popular in the US anymore but as I think about it this isn't all that dissimilar to how things were in the two decades preceding Frank Shorter. Although the US wasn't as far off the world leaders we weren't winning on the international stage that much until Shorter won in Munich and that kicked off a decade and a half of top finishes by Rodgers, Salazar, Beardsley, Greg Meyer that petered out after the mid 80s. Other than Hall and Rupp there is little to no depth and Americans are almost never a factor in major marathons.