I suspect that prep time has caused the dungeon to fall out of favor for many DMs. There are a lot of tools out there now, however, that greatly reduce that time.Dungeons are great. But they take too long to get drawn up and to maneuver around (the dungeoncrawl part). Meanwhile, just adventuring out in the world only needs a few set pieces drawn out and the rest described.
DMs making it into a plodding exercise is a separate matter and one that I imagine turned a lot of players off over the years.