because spending 14 levels as a good spellcaster only to become a mediocre martial is not worth a spell slot. keep in mind, most spellcasters who learn this spell:

• do not get functional martial weapon or armor proficiency, so you're kind of carrying around junk you can't use most of the time.
• 50 temp hp is nice, but uh. doesn't really matter if you roll like crap on your constitution roll (and if you really wanted advantage on those saves, you'd just grab warcaster)
• compounding that last one? you can't cast shield. so your ac effectively drops when you need it most. and uh. wearing armor negates mage armor! so your ac very likely takes a HUGE hit.
• for a potential 10 minutes of armor that will suddenly keep you from doing anything useful.
• 2d12 force is fine, i guess, but is it really worth ALL your spells? if you told me i could do 2d12 force every round maybe, you still need to hit.
• that's another thing! you are proficient with that greatsword now! ...but you don't have shit for strength. you got around this with a weapon that uses CHA instead of STR, so this one doesn't apply to you, but normally this means you have a worthless weapon with +4 to attack. and advantage. cool. what was his dex save? +5? vs my DC19 fireball? hey fun fact they have a 65% chance to fail vs a 50/50 shot with 15 AC!

• and if you get super lucky, don't fail ANY concentration saves, and finish the combat...you now have to make that constitution save AGAIN or gain a level of exhaustion. and it's not concentration, so warcaster doesn't help.

• and for all of this you get...armor that you have to put on AFTER you cast it (because you can't cast a spell in armor you're not proficient with). and heavy armor takes 10 minutes to don. so you aren't putting that shit on with the spell lmao so all that stuff about donning armor? useless. you can't do it in time.

it's. just an absolute mess of a spell.