Because there are several definitions for the term and how it is used depends on how, where and when it is used. One definition is "a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena" - that seems to be the definition that best fits how you describe the term.

However, other definitions include "a belief, policy, or procedure proposed or followed as the basis of action" or "an ideal or hypothetical set of facts, principles, or circumstances - often used in the phrase 'in theory'" (This is from the Merriam-Webster app on my phone because it's early and I don't feel like going to the bookshelf and pulling out the actual book, but I feel like it's valid.)

So, your use of the term is valid when discussing the "wave theory of light" (ok, it should be referred to as the wave hypothesis of light on account of the whole wave-particle duality thing but...let's go with it) or the "phlogiston theory", or "theory of evolution." But, my use of the term, and the use of the term in "non-scientific" contexts is also correct. In theory, people can use the word in a number of theoretical ways. They can also use the term in hypothetical ways. That's just how the language and usage has developed.

The term is not for the exclusive use of "science".

Steve