Why does Scrooge keep the carol in his office?

I'm assuming that you're referring to Scrooge's habit of keeping the coal-box in his office. The simple reason for this is that he's tight-fisted and doesn't want to spend any more money than he must on keeping the place warm. Scrooge has no problem with working away at his desk all day in the freezing cold. He doesn't even light the fire when he gets home, preferring instead to sit by the empty fireplace wrapped up in a blanket.
The same cannot be said for Bob Cratchit. He really feels the cold as he slaves away in the tank all day with only the light of a single candle to keep him warm. Scrooge is fully aware of this, but being such a mean old miser, he couldn't care less. He knows that if Bob had his way, he'd put a lot of coal on the fire to make the office nice and warm. To Scrooge, this would be an expensive extravagance, so he keeps the coal-box next to him where he can keep an eye on it.