Why would my doctors have induced a 3-day coma after I had a post-operative stroke?

Common treatment. If is a common treatment of brain edema (swelling) that reduces the metabolic demands of the brain.
Protect the brain. When you have a stroke, there are parts of the brain at risk of dying that can be salvaged. By inducing a coma, the physician decreases the demands of those "at-risk" areas and in some cases is able to minimize the extent and impact of the stroke.
To save your life. Therapeutic coma reduces brain metabolism, brain swelling and prevents seizures. This measure saves brain at risk for damage or destruction. Good work by your doctors. And good results for you!