"Top Tier" endgame content will never be alliance-based. If you managed to reach Turn8 or Turn9 in the first few weeks, you'd see how strictly specifically designed the enrage timers were. Alliances allow for weird possibilities the devs may not be able to account for, like 1 tank tanking 13 mobs and 20 DPS zerging passed a bunch of phases. It's just not as strategic. They like small content since it forces every member to feel special by giving them all important little duties. 24 man would be nothing but colors and shapes and icons all over your screen or just one mob that does a lot of AoE and is boring and a joke.