Probably a combination of NIMBYism and poor land-use planning. Back when many of these landfills were created, they were on the outskirts of urban areas, where "outskirts" = 15-20 miles away. After the wave of suburbanization in the 1950s-80s, suddenly those outskirts are right next to someone's home. Now it's a crisis.

Most people's conception of what land is available is limited to their local metropolitan area, and they have no idea just how abundant land is in the hinterlands. You could probably bury all the nation's garbage in the craters left inside the Nevada Proving Grounds, which is a radioactive wasteland anyway. And shipping contains of garbage 500 miles from LA, Vegas, and SF to Northern Nevada looks a lot more appealing when you've gotten used to shipping it 10,000 miles to China.