I grew up within spitting distance of Merion. As a teen, I looped at the local CCs, and even worked in the kitchen/dining room at Merion one summer. Merion is sumptuous with history. And not just the well known stuff, but decades of great names, and their sidelights, lowlights and highlights as they challenged the East Course. As I understand it, the caddies are still trained with knowledge of the stories passed down and can still act as somewhat of a historical guide as you play, if you ask.
You can stand and see exactly what Jones saw as he teed his ball to win the hole and US Amateur, complete the Grand Slam and never swing a club in competition again. You can also putt from the exact spot that Walter Hagen putted a ball out of bounds.
There are several reasons the big boys don't play there anymore. The first one is money. I'm pretty sure the second reason is money. And I'll have to check, but I believe the third reason is money. Merion doesn't have the space for the PGA corporate circus tents and Lolapalooza Show. It is surrounded by estates, houses, Cobbs Creek, and the P&W tracks/Haverford Rd on the north. And the course was designed for golfers.
It was said that Nicklaus singlehandedly obsoleted Merion East in the 60-70s because of his distance and the height he could hit the ball. I never understood that. Because as Nicklaus blasted the ball, Trevino carved his shots to fit the course as Hugh Wilson had designed it .
http://wpdilworth.com/Merion%20History.htm
I don't know how you can obsolete a course upon which you got your ass beat by a shorter, lower hitting ballstriker.
Merion still hosts many Amateur, Juniors and Qualifying events.
Charlie