There’s a key difference between everybody be still and nobody move. One commands people to do something, the other commands them not to do something.
Everybody be still tells people to stop moving. Presumably, at least some of them are moving, and the person issuing the command wants them to stop moving about. The people are being ordered to do something--to stop moving. You could replace the phrase with just be still and have the same effect.
Nobody move tells people not to move. Presumably they are in position, and the person issuing the command wants them to stay. They may not be moving when the command is given. The command is not given to nobody, it’s given to everybody just like the other command, but it’s in the negative because the command is not to do something. Unlike the other command, you could not simply replace the phrase with move; you need to put it in the negative--don’t move.
The literal effect of both phrasings are the same, but they are constructed quite differently. It’s the verb, not the pronoun, that makes the difference.