Why is a rhombus not necessarily a regular polygon?
A rhombus does not have to be equiangular.
A regular polygon must be equilateral (all sides the same length) and equiangular (all internal angle sof the same size).
A rhombus has 4 sides of equal length and opposite angles are equal but not all angles are equal. A rhombus may be shaped like a diamond. A rhombus that is equiangular is called a square.
A regular polygon needs all sides and all angles to be equal, whereas a rhombus only needs all sides to be equal.
A rhombus is a 4-sided polygon in which all sides are of equal length. While this fulfills one of the requirements of being a regular polygon, it need not have all of is angles be equal as well.
For example, this is a rhombus which is not a regular polygon:
Its sides are of equal length, but it clearly has different angles.