Because you only require 1 tank for every 8 people in 24 mans. All other content requires 1 tank per 4 people. So tank demand decreases by 50% and DPS demand increases by 25%.

On a side note, I really wish the standard party was 5 people. It feels like so many people roll tanks just for faster queues but they don't actually want to tank at all. If 5 (light)/10 (full) parties existed instead of 4/8, there would be a lot less pressure for DPS to roll tanks.