Because the main reason for companions' existence in this game is to f*ck em, and according to sources, dwarves, gnomes and halflings are the least "sexy".
We have Lae'Zel, but they made her hot too.
Look at DOS2 - we are not "dating" our companions, and suddenly we have a skeleton, an overgrown lizard and a dwarf joining the party.
I'm 66% sure of this statement.
