was reading up a bit on "The Fountain", seems to be a new trend to not use CGI:
"Instead of using CGI, Aronofsky chose to do the special effects for the film by using micro-photography of chemical reactions on tiny petri dishes. He has said that CGI would take away from the timeliness of the film and that he wants the film to stand the test of time." (IMDB.com). (might also have something to do with it being a hell of a lot cheaper too)