Actually, it makes a lot of sense that 24-men content is easier than 8-men content.

a 24-men alliance is harder to ORGANIZE and recruit than a 8-men full party, yes, but the difficulty stops the minute the alliance steps on the dungeon. With each individual player only being responsible for 1/24 of the total sum of contribution to the group effort, there's much more room for individual players to make mistakes (or even die) without compromising the boss try as a whole. That in and of itself is a factor that makes 24-men content inherently easier.

In other words... When an individual player screws up, that has much more impact in a 8-men full party than on a 24-men alliance. People on 8-men full party content have a much smaller margin of error.