Spectacularly bad production
This thing was the most spectacular waste of good-to-wonderful acting talent ever perpetrated. When many of the actors underperform like this, who is at fault? The director? Even the outstanding actors who performed well (Sean Connery, Henry Fonda, Trevor Howard) couldn't come close to saving it.
The special effects and miniatures were ridiculously amateurish even by the standards of a decade before. Just compare the cheap plastic toys and unconvincing effects to the brilliance of 2001: A Space Odyssey (1968), Close Encounters of the Third Kind (1977), Star Wars (1977), and Alien (1979).
Heck, as well as being a 1,000 times better movie in general, the effects and miniatures of Forbidden Plant (1956) had the pants beat off this thing. Destination Moon (1950) was better than this. This was closer to Buck Rogers from the 1930s.