But with Heavy Mil-Spec locking in at merely 35,000 ¥ it suggests to be quite inexpensive, considering the fact that each armor has to be custom fit for ist wearer and can't be mass produced.
So with that price tag it's hard to explain that such tech isn't already in use for security vehicles.
There's a few reasons.
1) Size/Mass. Armoring a person takes a lot less material than armoring a vehicle, and the weapons that are used against people are generally smaller than those used against vehicles, anyways. So armor for vehicles will cost more.
2) Numbers. There are FAR more military soldiers than there are military vehicles. Even so, the availability on milspec armor is very high.
3) Security. Simply put, while one person, or even a unit, in milspec armor is a definite problem, that unit supported by a milspec APC is deadly dangerous.
4) Restrictions. No one wants milspec armored vehicles to be out in the shadows. Absolutely nothing good comes from this equation for any of the people who have said vehicles.
5) Purpose. Milspec vehicles are purpose built machines. A milspec APC is not going to have an upgrade to make its armor hardened, it will have the hardened armor to begin with, if it can. This is the difference between a main battle tank and a Bulldog that someone slapped a crap-ton of armor plates on. They simply don't make hardened armor upgrades. You want that, you need a full build in a factory where you can bet there are at least twenty spies and informants from at least ten different organizations, making sure they know damn well who is getting ready for war.
Of course, the biggest reason we haven't seen anything like that so far is because of the type of game Shadowrun is. While there will eventually be a military/mercenary book, continuing the tradition from War! all the way back to Fields of Fire, the focus of Shadowrun is not full scale military engagements. In fact, if runners are ever going head to head against the full weight of a national military, they should have run a LONG time ago.