What would happen if one or both of the party conventions were bombed with massive loss of life? Suppose the nominee and his VP-nominee and the other major candidates were killed, along with many of electors.
Would the issue go back to the people, or would you get a deal done in a smoke-filled room?
“Please wait, the Department of Homeland Security will be right with you.”
Nitpick, Presidential electors don’t have anything to do with conventions. You’re thinking of delegates. Seeing as how the conventions are in late August and early September there wouldn’t be enough time to repeat the primary cycle. The party leadership would likely choose the nominee, but it certainly wouldn’t be in a smoke-filled room. 
In 1972, Thomas Eagleton resigned as the Democratic candidate for Vice President 16 days after he was nominated. The Democratic National Committee met and chose Sargent Shriver to replace him. I don’t think the rules have been changed since then.