Zor'lok was one of the worst boss design ever in HM, before the several fix/nerfs. So terrible boss AND difficulty curve totally screwed up.
Garalon was such a gimmick fight that most people really liked him or hated him completely.
Un'sok was a vehicle fight.
Added to that that the three other encounters weren't particularly flavourful (even if I personnaly really liked Mel'jarak.)
Also, plenty of guilds used an exploit on Mel'jarak, more or less killing the race. (Yes, Midwinter, I'm looking at you. )