Forbidden Planet is not a movie which generates refrigerator moments. It's tight; there are no holes in the story and it fits together nicely. Having watched it again recently I realized that if that movie had been given a proper orchestral score, and if the sound effects hadn't been 1950s gimcrakery, it would have been a knockout movie for the entire SF genre. Interstellar, sixty years early.
But it did occur to me, a few nights ago, that there was something the story did not tell us. It's not a failing; it's just beyond the scope of the story being told.
Were the Krell a spacefaring race?
The movie doesn't say. What it does say is that at the time of their demise, the Krell had been civilized for a million years. They'd conquered a lot of problems, including their own animal natures (so they'd thought) and had gotten to the point of building their massive machine, psionically operated, to make the satisfaction of every desire a thought away. Now, being able to build something like that implies an extremely advanced technology; and somewhere between "bang the rocks together, guys" and the construction of that machine, space travel seems like a given.
One would think that, after a million years, the Krell would be all over the place, not confined to one dinky planet. And even after 200,000 years, there would have to be relics of that civilization strewn about. The fact that the vast machine still runs inside Altair IV is itself enough to show that much. They could not have made just that one machine self-sustaining; certainly there must have been other installations which used that technology.
I don't think they would build machines like that one on every world and turn them all on simultaneously; that's bad engineering and the Krell proved themselves to be excellent engineers. (Proof: the big machine is still running without any sentient intervention, 200,000 years after it was turned on.) But the Krell are gone, and have been gone for a long time.
There are at least two possibilities.
First, that they did settle on other worlds, but for some reason those other worlds perished at the same time. I can think of some ways this could have happened. Alternately, the other worlds struggled onward for a time but also died out, and either these worlds just haven't been discovered yet, or else the evidence of the Krell presence there was eradicated by time, as it was on the surface of Altair IV. (The movie doesn't really describe the extent of the human presence in deep space, either.)
Second, that they couldn't travel in space, for whatever reason. My preferred reason is what Larry Niven refers to as "biorhythm upset", the case where an organism can't handle a change in the length of its day/night cycle. Having evolved on a world with a specific length of day, the organism cannot cope with changes in that schedule. "Jet lag" might be fatal for such a creature; certainly, living on a planet with a longer or shorter day would be. But there could be other reasons; maybe they couldn't tolerate zero gravity. Perhaps FTL was bad for them. Maybe they were herd or pack animals, and required the presence of other Krell, too many for a practical spacecraft. Maybe they couldn't take acceleration above surface-normal for Altair IV. (The shape of their doors suggest they were as wide as they were tall. That could be it.)
The thing is, the movie doesn't really give us any information at all about this. The story concerns what happened on Altair IV 200,000 years ago, and 20 years earlier, and "right now"; whether or not the Krell ever had colonies on other worlds is utterly irrelevant to telling that story, and the storytelling style in the 1950s was not to bog things down with too much exposition, anyway.
Ultimately the question of what the Krell did in space is irrelevant, because their civilization did end in a single night 200,000 years ago. But it might make for a really good story, even so.
Example: Krell on a colony world stop hearing anything from the homeworld and they send a ship to investigate. They get a fragmentary message from the ship and then nothing more. So they send another ship. It never reports back, but a few months later the first ship limps into port with a skeleton crew from the second. Based on what the report is from the traumatized remainder, a Krell official races to stop the activation of the colony's Great Machine--being started a couple of years behind schedule due to this or that--but he gets there a few moments too late....