Yep, I’m pretty damned certain that an aging McCain picked the attractive, young, energetic, charismatic governor because she was...well...young, attractive, energetic, and charismatic. Which not so coincidentally are all qualities that McCain himself lacked.
This really isn’t rocket science.
You left out the fact that she's also conservastive. If he hadn't picked a conservative as a running mate, many conservatives would have gone fishing Election Day, and McCain would have had no chance whatsoever of winning.