There is a good rational reason why simpler theories are more probably true: they are less probably tuned for the already existing evidence.

For example: Even if Ptolemy's circles made predictions that were equally predictive within that era's achievable precision of measurement. Those circles were tailored for that specific situation. Even if Kepler's law was quite ad hoc, Its simplicity could indicate that it had more substance, since it was not tuned to the given evidence in such a cumbersome way.