Explain why there are usually many possible theories for any set of phenomena?
It depends upon what "phenomena" you are talking about. Many "phenomena" are perfectly well-understood and are explained by a single, universally accepted theory. There are NOT "usually many possible theories for any set of phenomena".
Actually, there can be many hypotheses. Out of that group, then a theory can be worked out based on observations, testing, and other factors. https://upload.wikimedia.org/wikipedia/commons/1/16/Scientific_Theory_Flowchart.png
People are different and therefore have different ideas
Because until a theory is disproved, it can still stand as a possibly valid theory. (More accurately, unless it can be disproved AND people can actually *understand* the disproof!) Cheers. +++++