My guess would be that Lucas felt it was necessary to kill Padme to explain why Luke and Leia were split up from each other and their mother. Now i dont truly think it was necessary, I dont think that it was even necessary to have Padme pregnant during the film, well at least toward the very end...all that was needed is to have Anakin not know about it and/or him to believe she died.

But since Lucas went the route he did (and I believe its another one of those in your face look more direct connections from the OT) and decided to have Luke and Leia's birth on screen he was then left with the explanation of how/when they got split up...and like so many things he did he seemed to have not watched the OT (or have any memory of its events) before he went and wrote the PT.

-Darth Simon