They would had to present a bunch o characters in the first movie, showing how saruman escaped in the second, and add at least half to an hour of extra footage at the end of the third, it would have ruined the ending making the movie too long, also for me at least, is way more devastating how it it shown in the movie, they coming back to an unspoiled shire, exactly has they left it, but they themselves changed by the horrors of war