Examples from the Golden and Silver Age of comics
Horror-Tales of the Unexpected
Sci-Fi-Lost in Space(This comic came out BEFORE the tv series and the characters are far different)
War-Sgt. Rock
Western-Jonah Hex
Romance-True Love
Kids-Hot Stuff, Richie Rich
Girls-Katy Keene
Teen Humor-Archie, Sabrina
Take al look at DC and Marvel now.
Well Modern Horror (Bronze age) actually go back to Gold Key who was doing those types of comics allt he way back in the 1960's. You are right that the genre of comics in general has shrunk but for some of the above there are reasons.
Wastern - burn out pure and simple.
Romance - what comic market it had was killed by bad soap opera stories.
Kids - done by Disney LONG before Hot stuff or Richie Rich even existed and as a kid I found those character boring.
Girls - the marketing morons cannot seem to get the idea that girl focued stuff sells and so go as far as to mangle girl focused stuff like Cardcapture into a mess to make it boy focused.
Teen Humor - uh other than the digests when is the last time you have even seen a Archie comic? AFAIK they are bascily living on past stories like DC did near the end of the Bronze age with their digest reprints.