Like 99.99% of the other vehicles of its day.
Some units made V hull armor packages for them - but non of the Up Armor Hummers did well due to the additional weight.
Ideally you design a vehicle with the armor package from the ground up.
So everything is design to take the load, and the protection is optimized for the weight.
When designed with it armour becomes the chassis and body structure as well. When adding armour after the fact you already have a chassis and a body that you need to keep so that equals less protection for the weight or more weight for the same protection. Normal vehicle bodys also don't have the strength to hold up heavy armour so you would need to build framework to attach it to the chassis. This is why i imagine the MSVS SMP uses a removable armoured cab instead of bolt on armour