Blizzard operates in a free market. Average consumers don't care that much about security until something happens, then they forget. Therefore, security isn't a priority to Blizzard. It is really as simple as that.
Priority =/= invulnerability.
They care about security if only because they lose a lot of money spending time to fix up hacked accounts, and this will be even worse now they have a real-money auction house. In WoW they can just rollback items and if they duplicate some that were sold off by attackers it's no big deal. If they start duplicating real money then they need to pay out on it.