I wouldn't say controllers have low defenses. Maybe compared to a plate-wearing Paladin, yes, but compared to non-defender classes, they're actually comparable and have the capacity to be better.
The Wizard does have a high AC build. The rest are comparable to sub-par.
The issue I think is that none of the controllers - with the sort of exception of one Wizard Build and the sort of exception of the druid - do their jobs very well if they get stuck in in melee.
Controller Dependencies seem to be:
Needs defender to keep spanning ranged attacks and prevent too much hit point attrition.
Needs strikers to actually finish off enemies they've weakened.
Needs buffs from leaders for optimization. Everyone needs healing.
I need to find the DMG quotes on this issue, but that seems to be the basic breakdown thus far.
Most of the classes have 'secondary roles' that help break their dependencies - Paladins can heal a little, Fighters can do some hefty damage, and staff wizards can get some great AC and then spam thunderwave - but the core class structure still needs to reflect the dependencies.
Look at Druids, for instance, they get some great melee powers, but while they are using those melee powers they are explicitly forbidden to use their basic controller attacks.