yeah... exactly.

I blame that damn plumber! >:(

Because Nintendo advertises themselves as such.

WiiU:
http://www.gamesindustry.biz/articles/2013-12-18-nintendo-targeting-wii-u-marketing-to-kids-families

hell they went even farther with the 2DS:
http://www.polygon.com/2013/8/29/4671052/nintendo-2ds-targeting-demographic-of-very-young-kids-says-nintendo

Other factors being that their games are never or just barely going for a realistic artstyle and even some that do (looking at you Zelda) don't have things such as voice acting. I can't take that serious since I feel like playing a LEGO game.
Many of their games either don't have any story at all or just the same old simplistic "your princess is in another castle" plot. Games like Mass Effect, Heavy Rain, Bioshock, Fallout and so on make you think about history, technology, political systems, relationships, philosophy etc..

Obviously nothing of that means that it cannot appeal to other groups as well and I do like Mario, ... Wars and Pokemon, but it's still mostly a company that centers around children.

While Nintendo's marketing style is mainly targeted at children, their most ambitious fans are really above 18-20 years old and their games are suitable for everyone. It is just there are a lot of close minded people who reject something on first sight or base their taste in games/films/music on their friends or related people.

You missed out on a hell of a lot of amazing games.

- Jade Empire - Sid Meier's Pirates  - The Chronicles of Riddick: Escape From Butcher Bay - Ninja Gaiden Black- Project Gotham Racing 1&2 
- Forza Motorsport - Panzer Dragoon Orta - The Elder Scrolls III: Morrowind - Crimson Skies: High Road to Revenge - Doom 3 - Oddworld: Stranger's Wrath - Far Cry Instincts - Unreal Championship 1&2 - Jet Set Radio Future - Dead or Alive 3 - Deus Ex: Invisible War - Steel Battalion - Amped 1&2 - Otogi 1&2 - Shenmue II - Outrun 2 - Gun Valkyrie - Crazy Taxi 3 - Splinter Cell - Blinx 1&2 - Super Monkey Ball Deluxe - Rallisport Challenge 1&2  - Mech Assault - Star Wars Battlefront 1&2 - Kakuto Chojin - Crimson Sea - Half Life 2 - Azurik - Kung Foo Chaos - Moto GP - Tron 2.0

Not even including Halo and Fable.

And Xbox had (much) superior multiplats with extra content. Comparing SSX on PS2 and Xbox is like night and day. I know everyone's is hating on the XBone right now (probably deserved), but the original Xbox had AMAZING games. Please don't play it down like this.

Because they've made practically 0 M-rated games. I seriously can't think of a single M-rated game they've ever made of published other than Perfect Dark and Eternal Darkness. Everything else is usually E, E10+ and occasionally T.
Also, Nintendo almost exclusively markets they're systems towards a younger demographic. I dare you to try to find a single Wii U commercial not aimed at children and families.

Because vast majority of nintendo games are made for kids. Simple as that.

Well, it might have to do with the Color TV, Game & Watch, NES, GB, SNES, GBA, GC, DS, Wii, 3DS, Wii U, and 2DS. Also, no I don't consider them for children, they're rated E for everyone.

what Nintendo think: market it so children will drool on it and they will beg their parents to buy it....



what 10 year old kids: pff Nintendo games are for kids! I want call of duty!




what college student thinks: POKEMOOOOOOON!!!!!





what parents thinks: why do I have to buy that for my kid, he already have candy crush on his phone

^ This.

And while people love to point out Wii easily won the generation, which it did, statistics also show it as the least played console.  That's another reason we "quickly came up with other crap" to explain why people don't play their Wii: People bought on hype then lost interest, especially casual / non gamers.  Beside polls also indicate that to be the case.