Ease of movement.

[You must be registered and logged in to see this link.] I am BAFFLED by how the trooper at 2:32 is able to bend like that? I have a HIPS TE2, I'm 6' 3" which should leave plenty of gaps to bend, but I am completely unable to bend any further than to pick something up from a stool or similar.

Thin, flexible armor does the trick. I can bend down in my 1mm ABS TM. Not as extremely as that TK does, but I don't wanna risk too much.
HIPS will crack, at least my 1,5mm HIPS armor did in loads of places. No cracks with ABS so far!

Also don't forget for filming they were not worried about ruining the suits, coupled with flexy plastic = a wide range of movement.

the bend doesn't look that crazy to me either. his torso stays mainly straight, so I don't see how there would be much stress on the suit in this position.