There is a reason for this. So;

What if only Lily had died that night. What if Sirius never went to Azkaban. Imagine that Peter went to Azkaban. James, Sirius and Lupin helped raise Harry. And just imagine that Sirius didn't die in The Department of Mysteries, but Arthur Weasley did instead. And just imagine what the Battle of Hogwarts would be like.

Imagine......