Why are scientific explanations subject to change?

Science knowledge relies on experimental and observational confirmation. Where data is incomplete, new or improved data may well lead to revision of accepted science explanations. In situations where observations are fragmentary, it is normal for scientific ideas to be incomplete, but this is also where the opportunity for making advances may be the greatest. The core ideas of science have been subjected to a wide variety of confirmations and are therefore unlikely to change in the areas in which they have been tested. Science knowledge may change due to the development of new techniques for observing investigations (including new technologies), and also through new ways of thinking or framing the questions asked.

Because people make new scientific discoveries all the time. We don't know everything. We can only explain things in terms of what knowledge we have.