Maybe Father's Day is a factor in that too.
The final round of the US Open is always on Father's Day...
Unless one has a legit hookup (rare @ majors), attending majors has become a chore. (exception, Bethpage)
Factor in (i) the price, (ii) you really can't "see" the finish, (iii) the absolute sh*tshow as far as getting out/home, (iv) work the next day, etc...
...and double back to the price, again -- it's just not worth it. (IMO)
Bethpage is an outlier -- the LIRR & bus service in-and-out of the Farmingdale train stop cannot be beat -- seamless, just as good as it gets!!
That said, thanks to Andrew Cuomo's grandstanding in 2009, the US Open will not be contested at Bethpage again. (obviously the PGA has moved in to fill the void)