Dr. Awkward said:
Because Lathander's a good guy and good guys help people even if they're not total saints themselves.
A better question would be... Why would a devout priest of Good and Sun (since having a god is, in the default setting, optional) not resurrect her trusted companion the rogue, who has saved her life on many occassions, and who is a constant reminder of the importance of allowing others to have their own views, even if they contradict your own?
edit: I can just see it...
Townspeople: Oh, cleric of Lathander, thank goodness you're here! The vampires have been predating upon us, and we fear that our end is near.
Lathander: Hold on, aren't most of you Neutral? Okay, line up the good people over here, and the neutral people over here. Fine. Now, I'm going to protect the good people, but you godforsaken neutrals are on your own.