Actually, Bob Dylan died in that motorcycle crash...

but he was such a hot property that the record company couldn't let him go, so they searched for a replacement who looked and sounded like him. They didn't find anyone who could convincingly mimic his voice, but they found this Zimmerman guy who looked like a dead ringer so they hired him and changed the musical style to cover for the different sound.

To try to maintain the poetic quality of the new lyrics while bringing the music more into the mainstream they offered Paul McCartney an insane amount of money to come write for him and that's why he left the Beatles.