Why do chickens not try to get up if laid on their backs?
This phenomenon is called tonic immobility. And those speculations you've mentioned are wrong: they do get back on their feet, only remain in that state for a prolonged time, which depends on the distance and eye contact with whoever laid the chicken on its back, as stated here.

From this experiment, it can be concluded, that this behavior in which the animal is immobile is a defense against predation, because most attackers sense motion primary.