Question

Why are hypotheses and theories are always tentative explanations?

Answer 1

Hypotheses and theories are based on observations and expectations, but can not, or have yet to, stand up to rigorous testing of experimentation and observation. Until a hypothesis is tested, it cannot be considered a definitive fact. The same is true of a theory.
Typically a theory differs from a law in that it cannot be tested to be true all the time, and is based on observations that cannot yet be repeated and adjusted to test the theory's veracity.