Yeah Halo CE was the best, but also sold the least. Let's be honest here: 343 Wants the most buys as possible. One CoD fans learn that Halo 4 has perks they will give it a go (and if they ever decide to add Killstreaks? They will get even more CoD fans).
Look at sales: CoD has destroyed the competition. It's the ultimate casual FPS out there. People say Battlefield 3 is better. Well maybe it is, but it doesn't got shit on CoD sales. 343 Wants a piece of those sales, and while Reach did pretty damn well they want Halo 4 to do even better.
It's just that simple.