If there's going to be a lot of Fire types, there needs to be locations to put them in. Fire may be a starter type, but unlike Water and Grass, Volcanoes and Fiery caverns aren't your everyday sight. Bearing this in mind, when Game Freak do want to add several new Fire types, they have to get a bit creative, like sticking Volcanora in the Desert and making Litwick half Ghost so it could inhabit that tower. I've never had a look at how many of Pokemon there are for each type, but I'm willing to bet that types that fit better into more common terrain are higher up on the list.
That said, there does need to be a balance between realism and gameplay. Yes, Sinnoh was a cold region, but the lack of Fire types certainly hampered my enjoyment of the game.