Why would a scientist change their hypothesis?

In the face of evidence.

A Hypothesis is an idea about how something works or will work.

Once a test is performed (normally an experiment or a set of observations) then the hypothesis might need to be "refined" or changed to explain the new data.

Once a Hypothesis survives all the experiments that people can throw, by correctly predicting their outcomes, then it becomes the "Accepted Theory" for the problem.