Why do you think many people might be skeptical about psychology being a science?

The inability to predict in an accurate ways based on a deep and deterministic explanation for somebody's behaviour, and failing to explain individual and particular for every very single person.

Explanation:

Human behavior and motivation, are complex and often involve using a theory while disregarding another one less accurate.

In psychology, there are bodies of thought (for example psychodynamic approach) and often when psychology is applied to human behavior it contrasts with the results that were produced in lab conditions with animals. Thus unable to make clear predictions.

The psychology is based on questionable assumptions, that skeptics often attack. Most of the methodology uses a qualitative approach that contrasts to the quantitive method used by hard science like Physics or Chemistry.

We can then think of psychology as a field that gives some insight to our species but still, cannot evaluate the individual mind since everyone still will show certain actions and behaviors that are very particular to that person.