Why can't you get your drum to go back on after replacing rear shoes on your 98 Mazda 626?

The replacement shoes should be significantly larger than the warn units removed. The automatic adjuster has turned the star wheel adjuster open to compensate for that wear. Turn the star wheel closed and install between the shoes and the drum should go on. You will also need to adjust the brakes using a brake spoon through the hole on the rear of the backing plate after the drum is replaced.