One, Alec Guinness is a LEGEND, and not even Lucas is fucked up enough to mess with that.
Two, Obi-Wan died as an old man, as played by Guinness, and was never turned over to the Dark Side. The twisted logic Lucas is using to insert Christensen is that Anakin "died" as a young man, crossed over into the Dark Side, and maybe some kind of Force Statute of Limitations was in play. Although since Luke restored Anakin...

Aw fuck it, I like the first reason better. Sir Alec cannot be fucked with!
Lucas really fucked this up. Maybe he should just replace every head in the movie with Jar-Jar's.