A lot of players left due to loot rules.
Some people left because they got bored.
Some people left due to bug abusing in the beginning , and people selling thousands of golds shortly after the release.
Some people are logging in for raids only.
I personally left due to ban (ban was revoked) I played as pala healer,and needed on Necromancer Leggings against 2 mages (we had no spriest or lock) and pala healers had no other option but to need on stam/int gear. I got reported for ninja and i got banned. I was unbanned 4 hours later but it was already too late . I didnt want to continue playing .
Since then,i havent logged in single time.
World felt empty and leveling zones were dead even tho we had 12.000 players online. I had 4 characters,leveled in Dustwallow marsh, tanaris,stv, Hillsbrad Foothills , all big pvp zones (completed all quests in all of those zones) had 60 kills altogether. I got killed 8 times in total.