It was one of the first MMO's I've ever played where most people I knew/played with didn't buy the expansion because they didn't think it was worth the money.
A LOT of people have done the same thing, so the population is split between people still playing the original areas like Silver-wastes and such and the people who bought the expac to play the new HoT areas.
Since unlike WoW where a new expansion brings a new level cap so people "feel" the need to buy it, HoT didn't increase the level cap and instead just added a Mastery system. This system wasn't required for the base games lvl 80 content so people that didn't care for the new expac could continue to just play the game as the wanted without feeling that much behind.