I'm sure this is something many of you have asked, reading through my ultra-ultra long dragons and ardlin posts. However, there is a canonical reason for why there are so many varieties, outside of the usual "there need to be lots of different themed dragon-type creatures because D&D did it."
When a god creates a godtouched race, they don't simply design a single individual, clone them, and then let the resulting creatures run amuck. The god usually gives evolution a kick in the pants and increases the possibility of his or her race surviving by creating many different varieties at once, and seeing which is the most successful. The different varieties can of course interbreed, which might allow for a hybrid to eventually become the big daddy.
Because dragons are so difficult to kill, and so successful on their own, all different varieties have survived to the present day, though some dragons are more rare than others. Because ardlins are so new, there hasn't been enough time for inefficient or unlucky varieties to die off or crossbreed.
Some races, like naga and humans, have re-diversified due to spreading out over a wide area and having little to no interbreeding. In fact, legends of elves and dwarves come from varieties of humans that have long since bred into the rest of the population or gone extinct. Others, like ghrok and medusae, have no sub-races because they are on the verge of extinction.
And finally, some races, such as orchaein, are so new conceptually (I only created the plant-people a few months ago) that I've yet to really think about any possible sub-races, though there may be some I will add later.