Because irregular galaxies don't have shapes (in my belief they do) and spiral galaxies have a spiral shape and elliptical galaxies have a circle or oval shaped

Spiral galaxies have arms spiraling out from the central bulge of the galaxy and contain newer stars and has more gasses to form new stars.

Elliptical galaxies are normally formed after spiral galaxies merge. The formation is an elliptical mass of stars without any arms. When galaxies merge, the process destroys gasses needed to form new stars. So in an elliptical galaxy we would notice more older stars and less stars in the process of forming.

.. I hope this helps :) Get back to me..