They thought the dust was too loosely packed to support a spacecraft.
I remember that. It was the guy's theory that all we could see from the Earth, the mountains, the craters, the ridges and hills, were just friable, fairy-castle structures that would crumble with the slightest weight. The mare were supposedly nothing more than bottomless pits of loose, feather-light dust.
Even after the Surveyor and Luna soft-landers were down, he thought it didn't prove anything about the weight-bearing capacity of the surface. He suggested that the lunar module have some kind of markers or flares to drop down onto the surface before it landed, just to make sure that it would not sink out of sight in the landing attempt.
I forget the guy's name whose theory this was. I want to say maybe it was someone at Princeton (I was living in NJ at the time, so it rings a bell), but I'm not sure...