Scientific theories are just generalized statements that summarizes how the world works or how a phenomenon exist. It is based on verifiability and empiricism after which it becomes a law. When it's a law is when it becomes the truth, which means no matter the tests you conduct the results would all come put the same.
Theories change because change is constant and theories just like every other things in this world undergo changes. This is because theories are based on research results and this results are based on a wide range of influential factors such as the subject of the research, the researcher and the conditions under which the research is conducted. Hence the reasons why theories change.
If a particular hypothesis is tested to become a theory and other researchers in the same field decides to test the potency of the theory and they arrive at different results each of the researchers would postulate another theory based on the research result or modify the existing theory based on the result of the new research conducted. That's why theories change, because once researchers come accross new results or phenomenon, they modify the existing one.
In a nutshell, theories are not regarded as the truth until they have been proven severally through separate researchs and are regarded as laws.
"A theory is a logical-deductive system made up of a set of hypotheses, a field of application and some rules that allow consequences to be drawn from the hypotheses".
I mean, there's no absolute truth. And it's not that they stop being true, it's that theories get better.
The world changes, just as we do, so science does the same. Then begin to appear or disappear several factors that influenced that theory, and that perhaps at the time that the research was done served something. For example, it was believed that the planet Earth was the center of the universe and that the stars, including the Sun, revolved around the Earth. Why? because they had no mechanisms to realize, because simply the Sun "rose" and "set" day after day, just as you saw the stars in the sky, what would they know?
Or it could be the case that as the world evolves, or as destructive as the human being is, some material disappears and a theory ceases to be true. Not everything will depend on the scientist.