The word "dragons" sounds cool paired with "dungeons", the focus of the earliest prototypes of the game, and gives the consumer a sense of the sort of fantasy milieu they are jumping into. Naturally if it's right in the title (in the plural!) buyers are going to expect a certain number of dragons, as one of the two things they were most overtly promised. There was also an early need to fill monster manuals (there still is, but decades of source materials makes it a lot less of a scramble). Having lots of types of dragon helps. The general proliferation of dragons, naturally, does depend a lot on the campaign setting.
They are also common because people like dragons. They are the archetypical monstrous foe for the archetypical fantasy hero. Everyone wants to fight them at some point. Once a foe is relatively common it can no longer be mythical.
The Dragon as it is known in western fantasy literature basically begins with Beowulf, which introduces the cave, hoard, and firebreath, as well as makes it the final foe for someone with an illustrious adventuring career. It comes to us by way of the Hobbit, where Smaug is partly based on Tolkien's scholarly theories about the cultural meaning of the Beowulf dragon. Trying to match the dragons of typical Western fantasy after Tolkien with Welsh dragons, the dragons that early christian saints deal with in hagiographies, Eastern depictions of dragons, Greco-Roman dragons, or anything that isn't Norse/Germanic is not really going to line up as well as one might like. One would think something like the Greek myth of planting a dragon's teeth and a fierce race of men growing from the soil would be ripe for a game where people often harvest monster parts, but no.
I have no comparable explanation for this chromatic/metallic nonsense.