Why does smoking usually cause raynaud's phenomenon?

Nicotine constricts. Nicotine constricts blood vessels, so smoking can worsen raynaud's symptoms. Caffeine can also worsen raynaud's symptoms.
It usually doesn't. There are people with raynaud's where smoking makes it much worse, and people who only get raynaud's after smoking. But smoking causes blood vessels to constrict in all people, just not to the extent you see in raynaud's where fingers turn blue, white and/or red. In people with raynaud's disease or phenomenon who smoke, they are at higher risk of serious vascular problems - heart attack, stroke.

Related Questions

Raynaud phenomenon syndrome question. Is it really that or poor circulation?

See below. Raynaud's phenomenon describes a set of color changes in response to cold or stress. These are usually preventable and treatable. Poor circulation does not have those changes.
Color and location. Raynauds causes fingers to turn either white, blue and/or red usually in the cold. Poor circulation is different because it can affect palms and usually the border of the color change is a smooth transition. Raynauds usually has a distinct line between normal and blue /white.