Why are galaxies different shapes?

Galaxies are highly dynamic objects; that is, their shapes are subject to change. For instance, a galaxy that has been disturbed—that is, it has collided with another galaxy—becomes more spherical and round, whereas a galaxy that has been left alone and undisturbed for an extended period of time takes on more of an elliptical, spiral, and flat shape.

It is therefore essentially dependent upon the environment and surroundings of the Galaxy; if it had a turbulent past, it may have been more spherical or irregular, but after a few billion years, it may have shifted more toward a spiral shape.

It's important to remember that tiny galaxies rarely form any other shape than an irregular one because their gravitational pull is insufficient to create spiral or elliptical forms.

Galaxies have different shapes due to various factors, including gravitational interactions, mergers, and the distribution of dark matter.