Because the area is bruised. Basically you have has a mini operation. Think if you had had a mole removed, you would expect a certain amount of discomfort. Take arnica to reduce bruising, paracetamol 4 hourly and ibruprofen 4 hourly (stagered by 2 hours and make sure they are plain ie contain no other pain killing ingredients) and tomorrow rinse with hot salt water 3 times a day to keep area clean and clear from infection. Avoid extremes of temperature food/drink wise, don't smoke and try and chew on the other side to prevent anything getting stuck in there.
I had the same thing done, and it was quite uncomfortable for a good few days. Be patient, it will get better. If your tooth was cracked there may have been some infection in there too, as bacteria will have got in, so that may also cause a bit of pain (mine was chronically infected for 5 years!). Hope you feel better soon.