 Reeses outbid M&Ms for product placement. There is a rumor that  M&Ms were used initially, but the company asked for them to be  removed, suspecting that the film would flop. Another rumor is that  Mars the company that makes M&Ms did not want to be associated with  Aliens. It is worth noting, however, that the in the original  release of the book of the film, the candy that ET picks up is  specifically identified as being M&Ms.