Question

Why is a square always a rectangle but a rectangle is not always a square

Answer 1

For the same reason that a man is always a person but a person
is not always a man. 

And the same reason that a 5th grader is always a student but a student
is not always a 5th grader.

A square is a special kind of rectangle ... one with all equal sides.  If all the
sides are not equal, then it's still a rectangle, but it's no longer the 'special'
kind of rectangle.

Answer 2

A square is a special kind of rectangle, it is one where all the sides have the same length. Thus every square is a rectangle because it is a quadrilateral with all four angles right angles. But not every rectangle is a square, to be a square its sides must have the same length.