The bell is typically bell-shaped for two reasons, first because the circle is structurally strong and this allows bells to be struck with greater force than if the shape was flat or had sharp edges which would be more prone to cracking, further the circular shape allows a wave to travel around the bells perimeter so that standing waves can develop around the circumference of the bell. It is the resonance from standing waves that is responsible for the sound of the ringing.
And second the bell's shape makes the timbre of the bell more musically pleasing. The reason for the increasing diameter as you go from the top to the bottom of the bell is so that the bell resonates at different frequencies which can be tuned in a large bell so that you have what amounts to a complex musical chord playing when the bell is struck.
For example, a given bell might have a resonance at the fundamental, a subharmonic one octave lower, a minor third above, a fifth above, and a full octave above. The different diameter sections of the bell contribute to these different harmonics.
Bell construction is as much an art as a science. Here is a good online resource that describes the process of creating a large bell:
also the next chapter which goes deeper into the acoustics of bells: