Because most North Americans consider cartoons for children and would mark it off without ever actually watching it on that basis.
The thing is though that there are drama series for children. So this wouldn't fully explain it.
I think what it is, is that Comedy is just easier to get a success when produced for children.
In all my life I have only seen one child cartoon that was almost entirely a Drama and it still had plenty of comedy, which was almost exclusively contained in the comic relief characters who would show up at least once per episode. The rest were Sitcoms (Like 6Teen) or where the comedy saturation was THICK (MLP for example even if it is a drama... it is at least sub-comedy).
Now it also lacked Action as well. In fact one character "Knew martial arts" but did not use it once through the entire series (and unfortunately yes... she was "The Asian"... and even the fans groan at it)
and better yet it WAS popular. In fact the only reason it was eventually cancelled was because the toyline failed and not the show.
But that was the 80s, the 80s had everything that we desperately don't have now. Including female oriented action shows and Adventure shows (Goodness was the original MLP crazy! Even more so then the one we got now. I'd LOVE to see them even TRY to have the Skeleton horses of death now adays). It also had some really really bad shows too (I will be the first to admit... Biker Mice from Mars was pretty cheesy and bad).
I hate that I keep mentioning Jem and the Holograms (The non-comedy drama childrens cartoon that did have comedy but not a saturation), but ever since it astounded me by not being a terrible show, like I expected it to, I've been enthralled by how it can even exist. Actually the most hillarious parts of the show IMO were not even intentional and made to get a laugh, ohh the death traps in that show.
MIND you it had plenty of other things that were steriotypical and dreadful. You know those children shows that try to teach you a lesson? Yeah... Jem did that with all the tact of a sledge hammer. Lets just say they sing TWO SONGS about reading (the same one), Three songs about charity works and being helpful, and of all these the only good "learn a lesson" song was on comming onto strong.
The thing is though that there are drama series for children
Of the ones that exist NOW and arn't action or Comedy the only ones are for REALLY young children age 1-6.
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Also anyone remember the Magic Schoolbus? Honestly that was an amazing show and I think it should be institutionalised as a pernament part of television.