My Illogical Theory
Yep, another one of those crazy theories that no one can believe. xD
Let's quote PokemonDB, the site that no one can resist going to for their Pokemon needs (and which site is succeeded by Serebii, and Serebii is like my childhood. Oh yeah, the quote)
>Dragons are among the most elusive and powerful of all Pokemon. Nine legendary Pokemon are part Dragon type and four have legendary-like stats.
Nine. 9. Count that on your fingers - nine Pokemon are legendary Pokemon.
Also, now to think of it, dragons appear in myths, or legends. And 9/32 (just under one quarter) are legendaries. So, that kind of proves a lot. 32 - 9 is 23, which leaves it just 4 Pokemon behind the Ghost count.
That's the only real reason, but I assume why the Ghost population is so little compared to Dragons is because Ghosts aren't actually visible.
Like, if you were in the world where the Three Little Pigs incinerated a wolf, and two live gingerbread got decapitated by a creepy witch, there is 99% there would be dragons and ghosts in the world. Of course, you would see the dragons, but the ghosts are invisible.
Also quoting from the DB, it says:
>Ghosts are the rarest of all types, with just 27 Ghost type Pokemon in existence
It's the rarest of all types. They probably don't make many Ghost types because ghosts are invisible, and are hard to find. In the real world, how hard would it be to find an invisible creature? That's why the Ghost population is so little, because they want to make it mysterious 'n' unknown 'n' stuff.
Hope I helped. :)
Sources: Pokemondb.net/type/ghost and ./dragon