Why have only these three shapes persisted throughout time despite the broad diversity of bacterial functions?

Question

There are thousands of recognized species of bacteria (4000 have been identified to date), each of which are a little different in their genetic make up and
function. With all of that diversity, only three primary bacteria shapes have persisted throughout history; cocci (spheres), bacilli (rods) and vibrio (spirals).
Why have only these three shapes persisted throughout time despite the broad diversity of bacterial functions?‏