Pve specs

Earlier destruction was most popular pve spec for warlock, which is now cause i am seeing most of players playing demo?

If Demo is really popular, that would have to be because it's considered a raid staple in late WotLK.

Demonology is popular because it provides a very powerful buff to the raid and since warlocks are the least common class in the game, they're usually pressured into being demonology to avoid situations where they're the only warlock in the raid and can't provide the buff. It also does decent DPS.