I find the common cause of bad code is changing requirements. If someone is given the choice of a change that takes 1 hour or 1 week it's very hard to stick with clean code. Make a change and then undo it more than once, and it's easy to leave a few if (false) bits hanging around that don't break anything today.

Next biggest issue is a changing team. If you don't really understand some bit of code it's often safer to code around it than mess with it.