Some of it can be explained by trends and name recognition, as other comments suggest -- e.g. you get far more reactions to "We went to the Grand Canyon!" or "we went to Zion National Park!" than to "We hiked to Bluebell Knoll, the highest point of Dixie National Forest!"

But another part is definitely facilities. Being not originally from the US but having been to large parts of it, I was incredulous when I found out that BLM public land literally means just that -- that you can drive onto it and do stuff within reason; this is such a powerful concept that I -- and others -- often don't know what to do with it.

It's definitely an easier expenditure of mental energy to go to a big-name National Park, pay the fee, get the map, go to the Visitor Center, go to the big lookout and vistas, and leave with the magnet and the hat -- and I say this without intending to disparage this style of tourism. I lack the outdoorsy know-how and wisdom to feel at ease on the backroads of the desert southwest (and I live too far away from the backroads of the rugged northwest) where most of these facilities are, and it's a nice (albeit somewhat fictitious) thought that when you stick to the approved big tourist roads at big tourist places, that you're not completely alone in nature at the mercy of the elements.