Because they have 4 sides.
Well, you are supposed to call an irregular quadrilateral an IRREGULAR QUADRILATERAL. Haha. There really isn't any name for a regular quadrilateral. You just call it a quadrilateral or whatever the shape is. You could call it a REGULAR QUADRILATERAL.
i think you call it a irregular quadrilateral
A square
A quadrilateral is the name given to any 4 sided polygon.
A quadrilateral
A quadrilateral.
a square.
quadrilateral
a rectangle
Quadrilateral
quadrilateral
A trapezoid unless it is an isosceles trapezoid otherwise it can be known as an irregular quadrilateral