The irony of it all. I think Akatosh saw it beforehand that Alduin himself is saving the Dragonborn from execution without knowing it. If Akatosh didn't know, it would've been a little too close call. I think Akatosh just effing around with Alduin on this one, provided that Alduin later realizes he actually saves his worst enemy to come.
Had the main character died, had someone else become Dragonborn at Akatosh's will, I don't know. I bet this would've happened.
Oh, btw, Akatosh is Alduin. Discuss.