What is a reason a theory cannot become a law?

GREAT question! Too often we regard science as "absolute". But it's design is to ALWAYS QUESTION, and base answers on observable, repeatable facts.

At best, we recognize the usefulness of consistent relationships. The MOST stable of those we do call "Laws" in science, but that does NOT make them unquestionable by science!

In the strict scientific method terminology, everything is a THEORY. We conjecture how some things interact and try to devise experiments that support (NOT 'prove') those conjectures or not. The more often a theory is supported by evidence, the more reliance we can put in its use. HOWEVER, that does not mean that we ever can say that it is the final, correct interpretation!

Newton's "Laws" of Motion have served us very well for centuries, and they are still applicable in the macroscopic world. But we now know that they are not the correct form for all motion - Quantum Mechanics has replaced it. Quantum Mechanics IS applicable at both atomic and macroscopic levels. It is just too cumbersome to use in all instances.

Further, we cannot say even that Quantum Mechanics is the "final" accurate description of the universe's motions. We use the term "Law" to indicate a long-useful Theory. But it does not mean that it cannot be changed.