Probably so they can sell three times the merchandise. Actually, each of the villains did play a key role not only in the action, but in the theme of the movie, so the reason probably isn't as crass as merchandising. Though I'm sure the suits don't mind that aspect. Spidey 2 was terrific, but when it comes to comic book movies I judge them by the ten-year-old inside of me. The first movie gave me multiple goosebumps. Spidey 2 only once. And Spidey 3 not at all. My order of preference is the same as the order in which the movies were produced, but in my opinion the quality gap between Spidey 2 and Spidey 3 is greater than that between 1 and 2.