Because he gets all chopped up (started in II). He's mostly machine in the IV, V, and VI.
Somewhere in one of the first books (from years ago--with Lucas listed as the author) Obi Wan talks about how he and Anakin got into a fight on the planet Sith(didn't say why). Anakin fell into a lava pit during the fight. There wasn't much left of Anakin when he crawled out of the lava pit. Apparently the Sith Lords nursed him back to "health", in turn he joined them,etc.
My theory: Obi Wan and Anakin were fighting about nubile nookie, otherwise known as "Padme."
LOLOLOL