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.
I'm referring to an electric train powered by green energy—an ideal, not a current reality. As far as I am aware, shipping lanes are just as difficult to green as planes are, maybe even more so.
We know how to make very large nuclear powered ships. You could fit an astonishing amount of waste on something the size of the Nimitz, especially if it didn't need to make task group speeds.
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.