Why Phase-In Minimum Wage Laws?

The reason is clear.

The fact that politicians often "phase-in" increases in minimum wage reveals an awareness that they have disemployment effects. If such laws were as benign as they say, there'd be no need for a gradual transition. However, it's well-understood that increasing the cost of something, even if it is a person, decreases the demand. Who is most harmed? The least productive people. That means the people with the fewest skills, earnest the least, will be cut first. For all of the bluster about how minimum wage is supposed to help, this is a matter of enacting a policy to feel good: not to help. All that phase-in does is sprinkle the disemployment effect over a long enough period that it's less obvious but no less impactful.