I think it's because most people are followers and not leaders.

How often do we hear "we started, but everyone dropped". It takes some effort to keep a group going. It's not going to happen with poor leadership or lack of motivation to manage a group. It's like tanks in 4mans. People rather play dps/healer because they can follow, less responsibility.

I hope SE will give us two modes. One easy enough so it can be pugged. The other one with the difficulty we have now or maybe even more difficult. Give loot the same ilvl but other aesthetics (color).