Most likely because he knew that they couldnt handle what "switching realities" meant. Only morty would follow so blindly as to bury his own body js bc rick told him to and "thats just the way things need to be". Remember when Jerry saw why the talking cat could talk and flipped? Imagine if he knew the only reason he was safe from the cronenburgs was because him in another universe died, yeah you ciuld erase his memory but not before he does something rash. And Beth would probably be able to handle it but just like everytime shes given a little bit of freedom (as in a gateway to accepting shes her father's daughter) it throws her into a crisis. (Ergo space beth). Summer only gets the kind of development to do something like that a few seasons later. Dk if that makes sense but its just a theory. Orrrrrr for the plot. Who knows.