Normal is vastly easier than Elite!
Heck since Champions Normal is vastly easier than Hard!
Why would anyone group for Normal when they could simply solo it without ever taking any dmg?
Giving Normal the same XP as Elite would have more people running Normal yes but those people would be running it solo and groups would still be Elite only!
Oh and Hard simply wouldn't get run ever as there'd be no point running Hard - If you want the higher chance at loot you run Elite.
{The above is for Heroics - Epic dynamics are quite different!}.