I was looking over a listing of past champions of the US Open Cup and noticed that since the MLS has been in existence all of the winners have been MLS teams, with the notable exception of the Rochester Rhinos in 1999. No surprise there, the MLS clearly has plenty of talent. What DID surprise me however is that there are no winners at all from the old NASL. I can only assume that NASL teams did not participate in the tournament. I watched plenty of NASL games as a young man and an dead certain that had they played in the US Open they'd have won plenty. Even my beloved if not terribly good Washington Diplomats could surely have beaten the N.Y. Pancyrian-Freedoms (1980). Any football historians here know why they didn't?
I think the NASL was trying to position itself as a truly bigtime league with bigtime stars and felt the USOC was too minor of a competition to bother with. There were no nationwide minor leagues either like there are now, so they would have just been competing against regional amateur club teams (and each other), so they probably had a legitimate point. I suspect there was very little consideration even given to the idea.
The NASL was a pretty stuck-up outfit. They defied not only FIFA, but the USSF as well. They competed in something called the "Trans-Atlantic Cup" in the 80's, but nothing so lowly as a domestic competition. The American Soccer League ran at the same time. It was offically called a division 2 league. I am not positive, but I think some of the teams competed in the USOC.