Because of the horrible weather. Around this time of the year Rime Icing builds up very fast and if you don't activate the Pitot and Carb Heat then the Pitot Static System on most planes will freeze and it will disable things like your airspeed indicator and altimeter. These a very necessary instruments for any pilot and this is the worst time of the year for it.
And just for heads up, I'm saying this as a pilot to all of you guys who are passengers or who use airports and don't fly themselves. When your plane gets delayed or your flight gets canceled please stop bitching and moaning. We don't cancel flights just to piss you guys off. The PIC (Pilot In Command) has the last word if a flight is go or no go and weather changes very suddenly. If we cancel your flight it means that the risks of crashing and dying are far too great for the current conditions. We are looking out for YOU guys. We're not trying to screw you guys out of getting to Boston on time. We want you to LIVE to get to it.
/rant