Licensing in different countries can affect who owns the cover artwork rights. It can be cheaper and easier to simply license new artwork within the country of sale, not to mention make the book's cover art more culturally relevant and attractive to the buyer.

Publishers putting out new editions of the same book can "freshen" up sales with new artwork that reflects an event in relation to the book or its author. For example, the author's passing (which is often highlighted in the media, driving up interest in the author's body of work), or a publishing "anniversary" of a famous or infamous book.

You may also notice that books translated to film will often have "tie-in" cover designs that match the film marquees, in order to drive up interest and, ultimately, sales.

It's standard media industry practice (books, music, etc.) to use cover designs and redesigns to refresh and move more units, at least before the copyright expires.

Exactly what Alex says. I would add that publishing is also a bit of an exercise in ego (as are most things), publishers at least pay lip service to the notion that they are bringing something to the attention of the public, filtered through their finely honed notions of what is tasteful and important. When a book is bought by a publisher from another publisher (as opposed to being acquired and printed as a first edition), one cannot do much other than put a new cover on it to create the impression that one has placed their stamp on it.

I love pulp covers, those are awesome, thanks.

Alex has it right. A popular, widely sold book can go through many, many covers over its lifespan.

For example, the author's passing (which is often highlighted in the media, driving up interest in the author's body of work)

Our euphemism for that here at GiantEvilBookseller, Inc. is a "cultural event." Just us?

yup--what Alex said--each new edition, each different publisher, whim, total back catalogue redesign, an Oprah thing, a movie being made from it, some publishing trend it fits into, etc...

Different countries have different aesthetics, too. I've found books I loved that I would never have looked at if I'd seen the U.S. covers instead of U.K. ones.