Galaxies wind themselves up too fast.
Several years ago I wrote a galaxy collision program in which I am able to mimic collisions. When I run the program with only 1 galaxy having several hundred stars I was surprised to find that spiral arms would develop and go away only to develop again. Whether more sophisticated modeling than mine would uphold this view I don't know, but it seems to me that this might be the reason we see spirals. Comets disintegrate too quickly. This one is easily dismissable.