I think it has more to do with design then anything :/
Back then, when GameFreak actually PUT IN fucking fire Pokemon and just made the Dark Type, they had some true bad ass designs. Hell, to this day the only "cool looking" Fire Pokemon that are not starters are from the first gen.
Some other types don't have random bad ass designs for their non-starter/ legendary Pokemon. Like water . . . where is their bad ass Pokemon that fit those rules? Their just fucking fish most of the time . . .
I mean look at these
http://students.ou.edu/F/Matthew.W.Fenwick-1/charizard.gif[/ IMG]
[IMG]http://images1.wikia.nocookie.net/es.pokemon/images/4/47/Sneasel.png[/ IMG]
[IMG]http://pokemontrainers.lapunk.hu/tarhely/pokemontrainers/kepek/arcanine.png[/ IMG]
[IMG]http://projectpokemon.org/generation5/Zoroark.png[/ IMG]
You can clearly see the "coolness" of their designs and I think thats what it comes down to . . . these types are (fire WAS in the first gen) heavy in "cool designs".
Should I rest my case?[/QUOTE]
You've just proven my point. Kids' love of Fire and Dark types is all style over substance. Theme over utility. They love "badass" designs (i.e. aggressive, cool, counter-parental-values-but-in-a-Nintendo-safe-way)
Also, Water has goddam Gyarados, but one pokemon does not a popular type make.