This question already has an answer here:
Why can't I access SO and others through https? I believe it should have a https interface.
Hmm. I can't see a compelling reason for this.
Your login is handled through your OpenID provider, so somebody eavesdropping on the communication between SO and you would not be able to get your password.
As far as I can see, all interaction with the SO site is stuff that is visible publicly anyway, except for votes, flags and ignored tags and such. That is private info, but hardly so security relevant that encrypted communication would make sense. Is it?