Well, Lelouch has a harem, he's just not really interested and is too much of a siscon. But at least he dates Sheryl, steals the heart of 3 other girls (CC, Kalle and the princess) and admit he knows how they feel about him. It's still a harem setting nevertheless, just a more decisive protagonist who knows his priority.
In Tatsuya's case, his priority is the same as Lelouch, destroy his own insane family and protect his imouto.
I'd be really disappointed if the whole series only focus on Tatsuya x Miyuki when there are quite a lot of girls interested in him.