I'm not a publishing professional, but I've always seen the copyright notice as a fine print sort of thing. It's there for legal and informational reasons, but it's not critical cover information. I'm not sure I see what purpose would it serve to place it on the cover. The only possible benefit I can think of is making it clearer when the book was published. But that's not really necessary information for the cover, and sometimes books that have multiple editions will have multiple copyright dates.