Pretty much lol, getting a console for a kid is getting rid of your tv.

Also he can play with friends on a portable, online isn't the same for kids.

Pokemon, and only ever had one very powerful competitor sales and reception wise (the PlayStation Portable).

I think it's because Nintendo has always kind of been a mobile company at heart. Their games are typically simple and easy to play in short-bursts, which lends them well to handhelds. Plus, Nintendo has always given people what they wanted out of a handheld. Simple, fun, and accessible without fixating on superficial stuff like specs or "maturity". Nearly all of Nintendo's competitors in the space failed because they went in with the assumption that bigger and flashier = better, when in reality, people just want simple fun. The 3DS however, did struggle against the rise of smartphones, but a lot of that had to do with the mistakes Nintendo made with the system and not making itself clear why its different from phone gaming, which led to the system having an identity crisis.

The Switch however, instantly makes itself clear and distinct from conventional mobile gaming by being a home console you can take with you, and features like the Joy-Con that allow for experiences that can't be replicated by any normal phone or tablet. This is the reason the Switch is so successful, it's able to justify its existence in a post-smartphone world in a way the 3DS simply couldn't.

For whatever reason people seem to prefer colorful/cartoony games on smaller screen portables it seems like.

Even look on iOS/Android ... many of the top games have cartoony art styles that would be right at home in a Nintendo game.

For consoles, especially on the big screen, people seem to prefer more realistic graphics and they also expect a wider variety of content.

You can't wholesale be missing key genres/big 3rd party franchises on home only console. But on portable people are more lenient on library, if you don't have a Call of Duty or Madden or something it's not a big deal. On console though people want the variety and Nintendo has struggled with that since the SNES era.

Personally, I think it is because people that buy Nintendo products want simple, rather intellectually basic (and therefore cheaper) products. They want the convenience of playing on the go and they don't care about the worse visuals. Nintendo gaming is about having fun, not watching a visually stunning movie. Now, with the Switch, they can still appeal to this group, but at the same time offer a massive leap forward in tech for a handheld. The 3ds --> Switch is most certainly (unless someone can point out another time) the most massive technology leap ever in a gaming device.