Simple, smoking promotes blockages which will close your stents just as they would an artery which is most likely why you had the stent in the first place. For your health, don't smoke especially with heart disease.
To elaborate. Arteries, particularly those with lots of pounding and intersections are prone to damage. The tiny coronary arteries are connected directly to the largest artery in the body, the Aorta, so they get more abuse than other arteries of that size. They regularly develop tiny fractures which are very quickly repaired, repaired before the immune system and other factors have time to step in. The chemicals from smoking, which enter the blood, inhibit those repairs and slow them down. They also cause more damage not just due to the chemicals directly, but because smoking increases blood pressure. If the repair isn't done quickly enough, your immune system gets macrophage cells into the artery wall, and this causes inflammation. This then makes the fissure larger, allowing other blood cells to get trapped in there. This is the start of artery disease.
I hope this helps.