Another possible reason doing the rounds is that it's because Daniel Radcliffe will be in the middle of the Broadway run of Equis at the time of the orginal date of release. A play that shows him naked and having sex. One theory is that it's not the image that Warner Brothers wants associated with Harry Potter which is primarily a childrens film.
How much truth there is in this theory I'm not too sure as if it was going to be that big of a deal surely WB would have put a stop to him taking on the role to begin with but then again maybe they didn't know it was going to be so successful.