Why? Because he's afraid, that's why.

Chase, Curry, and Taber ask: Why won't Harper take Ignatieff on in head-to-head debate?

He has no spontanaety, isn't as quick as Ignatieff, and has far, far too many issues he'd rather dodge.  Lacking the skills of a relaxed debater, knowing he can't stick to scripted answers, there's no way he's going to let people see how poorly he preforms in situations he can't control especially when he has a clever, impassioned opponent he can't intimidate.

I never believed steve would agree to this in the end.  Especially if he couldn't frame it as a debate between him and The Coalition™ leader.  Ignatieff is no idiot and insisted that another debate with all the leaders would still have to be held.

So steve is backing up, and earns himself the Backing Up song: