Comfort
Skiing takes place during the winter. Typically this means it is cold outside. It can also be a very physically intense material so it can result in sweating.

Boots have to create a specific environment to protect the users foot and ensure comfort, so climate control is an important issue. Insulation is used to keep the foot warm by creating a barrier that prevents heat exchange. However, if a material gets wet this can spell bad news for a skier. Water transmits heat at 4 times the rate of air (which is oftentimes the barrier used, done by using light materials to create space).
  
The foot doesn't have any major muscles and is warmed primarily by blood flow. Bad fitting ski boots (as in possibly cutting off circulation) combined with cold temperatures perform a one-two punch that could quickly lead to danger.