They had content for 3 players that was scenarios. Those didnt really work. The main issue with what you are asking for ultimately equates to running with two out of the three pillars of WoW instanced combat. So any of the duo instances, will be hard for some comps, but easy for others.
Do you have to run with a tank and a healer? Or Healer and DPS? Or DPS and Pally Tank for Off healing? In order to make them a challenge, there would just be a crazy amount of imbalance. And in order to make it function for all the combinations, it would take a ton of time and financial investment into creating that mode. And that in itself is a deterrent.