David Slade knows why! Since he was the director, he is the man in "the know."
And when asked about the memorable tent scene, which baffled my coworker, who was confused as to why Bella wasn’t wearing a jacket in the midst of a blizzard, Slade simply responded:
“Poor planning on Bella’s behalf I would say. If you’re going to go up a mountain, you should wear a big coat. It’s just really silly to be honest. I wish she was wearing a hat though.”CNN