Why does the main door in a church face west?

Broadly speaking, in order to face towards both Jerusalem and the rising sun. Many churches are not exactly aligned though -- some used magnetic north to align, which varies over the years, and others are aligned to the location of the rising sun on the feast-day for their patron saint. Plenty are also just twisted around because of requirements of the plot of land, builder carelessness, historical whimsy, and so on.