The more people there are, the more leeway there is for bads to be carried. 8 is a good number to have challengive yet meaningful mechanics. The more spoons you throw into the soup, the messier it gets (see wod etc.).

Imo they could even add 4 man light raids to really challenge both one's individual and team contribution the maximum.

As mentioned, it's hard to optimize a 24-man so it proves a meaningful challenge for everyone. And see how many people were already mental about how hard it was for them to fit in a 8 person static? 24 man raids would mostly be a big FC thing.