the shadow of the Earth falls on the moon blocking different portions from our view.
the sun shines on the Moon at different angles at different times.
a different portion of the moon that is lit by the sun faces Earth at different times.
the Earth's revolution around the sun.
the moon's revolution around Earth.
the moon's rotation on its axis.
different amounts of the moon are lit by the sun at different times.
when the moon blocks parts of the sun.
the moon's rotation and its revolution around the Earth take the same amount of time.
the phase occurring depends on what time of day you are experiencing.