Besides they didn't just attack one planet, they started in Batarian space then moved on to the Alliance, then hit Palaven, and latter hit Thesia, before taking the Citadel when they found out it was part of a super weapons that could kill them and thus important again.
Why didn't they just attack the Citadel from the start? Their entire cycle by cycle plan has always been to arrive at the Citadel, kill everyone on it, extract all the data on the whereabouts of all the species who have used it and control the Mass Relays. Everything from leaders of multiple species to the information regarding their strengths and weaknesses are all still there in Mass Effect 3. Heck, it's the place where everyone is going, yet they don't attack it 'until' they figure out that the Crucible is part of a larger weapon.... which they should have known all along.