Why is a square always a rhombus, but a rhombus is not always a square?
It is important to work with definitions first.
A parallelogram is a quadrilateral with two pairs of opposite sides parallel.
A rhombus is a parallelogram with equal sides
A square is a rhombus with all the angles equal (to 90°).
Students often make the mistake of defining a rhombus as
"A rhombus is a square pushed over."
It would be better to say that a square is a rhombus pushed up straight.
A square has all the properties of a rhombus, with more properties -
A rhombus does NOT have all the properties of a square, therefore is not a special kind of square.