Creativity and originality require taking risks and it is more difficult to have a big hit with something new (which targets a new player audience -- or upsets the existing audience if a sequel is too different from the previous version) but what other reasons can you think of. Structural, political, technical... or is it really just pure business?
I do not want to exclude that there are innovative games (also sequels) out there. However I do think that there are certain creativity killers stopping developers from going really wild and create the games they would really want to do. Look at what is being promised about a game 2, 3 or even 4 years before launch and then look at what is left from all those ideas. There are exceptions but what is it that eventually kills those ideas?