Why is a rhombus not necessarily a regular polygon?

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.