While the landsknecht were a professional (maybe semi-professional) force they were not drawn from the societal elite. The majority of them were unarmoured and I'm guessing unfamiliar with armour in general. They could see the advantages of armour but, much like myself, would not initially think of things like arming garments. As such, when equipping themselves they focused on the hard shiny bits.
An additional thought about arming garments is that they are much more complex to construct than the wams the landsknechts were wearing. Having just finished making a wams I can personally attest to how easy it is. I am no tailor and even I have been able to handle putting it together. Four rectangles of various dimensions and some time with a needle and thread and you have a surprisingly good looking garment. Not having tried it I can only guess that an arming doublet is a little more challenging to construct.
Thoughts?







