Blizzard attracted a lot of new players with WCIII, so it makes sense for many people to end with WOTLK. Like a lot of people mentioned previously, Arthas was not the best written character that they had, but it is probably the most relate able. Human, rise and fall, destruction and the after effects. I think many people felt for the stories in Eastern/Western Plaguelands as well.
Most people quit around that time because the majority of that age group that played WCIII/Classic moved on to IRL stuff.