Largely because they're actually being sent to places like Afghanistan to make up for the shortfall in 'full time' military personnel. Signing up for the TA doesn't seem to much like such a good idea when you can end up in Helmand. OK, they're not on the front line but I don't think IEDs and suicide bombers make the distinction between paid soldiers and TA.
I don't blame them at all mind. The TA has always been there in theory to do the mundane stuff therefore freeing trained soldiers to do the fighting. Sending them to combat zones is stupid. They're not trained properly for it and they didn't sign up for it. If they'd wanted to do that, they'd have actually signed up for the army not the TA.
Jon