They are already using 500lb and upwards IEDs, there is quite simply nothing that can withstand that size of blast hence the number of KIA due to IEDs rising over the past year or so.
There is a sliding scale of where a vehicle can be:
Manoeuvrability/Visibility/Ease of use -----> Weight/Protection
Chuck in the fact that you now need a more powerful engine to do the same work as weight goes up and that entering/exiting a vehicle designed to resist blasts is not likely to be as convenient as one which isn't.
That thing clocks in at 7 tonnes? The Land Rover Wolf is 1.6 tonnes. You can see it's moved along the scale a fair bit.
Only so far you can armour a patrol vehicle before a tank starts looking preferable and tanks are pretty rubbish patrol vehicles. Awful visibility unless someone's sticking their noggin out the hatch which leaves him with very little protection.
Oh well. Media will now grab an even more armoured vehicle to demand as an upgrade when it turns out the armoured vehicle they banged the soapbox for still gets blown up by an IED the size of an oil drum.