Why are spiral galaxies the most commonly observed galaxies?
They are the most commonly observed from Earth, but not necessarily the most common (ellipticals are).
The exact mechanism for the formation of the spiral arms continues to puzzle scientists. Scientists think they could be a result of density waves traveling through the outer disk.
The formation of spiral galaxies is thought to be a complex process in which the stellar halo, bulge and disks are formed at different times and through different mechanisms.
The disks are thought to form after the primordial collapse event responsible for the formation of the spheroidal bulge and halo, possibly through the cooling of the hot gas contained within the halo of the newly formed galaxy.
Some very good additional resources and discussions here:
Nearly 70 percent of the galaxies closest to the Milky Way are spirals. New research finds that spiral arms are self-perpetuating, persistent, and surprisingly long lived.
Spiral galaxies make up roughly 77 percent of the galaxies that scientists have observed. However, they are not thought to be the dominant galaxy type. That honor goes to elliptical galaxies, which spirals are thought to ultimately degrade into.
Because elliptical galaxies are made up of older, dimmer stars, they are more challenging to spot. In large, in-depth surveys of patches of the sky, elliptical galaxies have dominated, leading scientists to conclude that they are prevalent throughout the rest of the universe.