The statement “force equals mass times acceleration” is newton’s second law of motion. why is this a law rather than a theory?

It states, rather than explains, the relationship between the variables.

Explanation:

Let's start by defining  theory and law:

Theory is the set of rules and principles that describe and explain a particular phenomenon.

In other words: It refers to the explanation of the functioning of things.

Another important point is that the theory is subject to changes as new evidence emerges that gives meaning to it.

On the other hand, a Law is an affirmation (something established) based on repeated long-term observation of a phenomenon that has been studied and verified.

That is: A law is present in all known theories and therefore is considered universal. In addition, a law can not be refuted, nor changed,  because its precepts have been proven through various studies.

Then, based on what is explained above, the statement "force equals mass times acceleration" is a law because it is a statement that exists because it was rigorously tested and verified, therefore it can not be refuted.