The armor needs mud and twigs and leaves and outdoorsy stuff to function. Ranger's touch does allow the ranger to call upon the elements of nature in order to summon needed magic to cast innate spells indoors, but Forest Armor is a bit different in that it's not YOUR spell knowledge - it's the knowledge and magic of the armor, which doesn't function when it's not out in the open environment, or is in water.

~ Haliste ~
The Forest Gnome of Silverwood Manor
ASGM, World Development