I have come to the realization that I could and would legitimately think less of a person between the ages of 6 and 25 who knew about Pokemon and never liked it. I'm completely serious. That's a red flag.
I can honestly see why people wouldn't like pokemon. The games usually are devoid of all but the most simple of stories with gameplay SO easy that you can literally play the game with no issue with the first 6 pokemon you catch.
And now if you haven't been following Pokémon for years you have to play basically with a guide open because pokemon typing often feels like the creators just took a cursory glance at the pokemon, saw one feature can entirely changed its type (DANG it Empoleon you do not justify your steel type... Neither do you Lucario but I assume you are meant to be Iron fist or something so it is a cultural thing. The worst is Flygon which is Dragon/Ground for no real reason except to give it 4x weakness to ice), some of the "what beats what" seems random at best and sometimes even contradictory. With moves that do not telegraph their use or type a lot of the time either such as "assurance"... and several hidden features that can often mean the difference between keeping and tossing a creature.
Also before you ask... I do like pokemon.
The gameplay is just a matter of good UX design. Easy enough that anyone who can read is able to play it; but with enough depth below the surface to make it entertaining over a long time period; Shinies, IV, EV, breeding, moves, ect.
As for the type system, I think it may actually be meant to be somewhat inscrutable in some cases. It thus serves as a mastery mechanic on two levels while still being easy for kids to learn. The first level is the simple 'x beats y' stuff. Just looking at most 'mons will give you a good idea of their type... Except for when it doesn't. And therein lies the second level of mastery: learning the types of the individual critters, especially those which are outliers. Even a kid can grasp these mechanics and thus become better at the game, while still occasionally being thrown a curve-ball when a seemingly flying dragon is a ground type that eats your thunderbolts for breakfast.
The stories are very well designed: targeted directly at the entry-level demographic of kids who are just of the age that wandering around and raising a team of critters to fight baddies and become the best is extremely appealing. Pokemon stories can mostly be summarized as a juvenile power fantasy; which is perfect for the juvenile demographic they're aimed at. People older than that will roll their eyes at it, while occasionally giggling at some of the more contrived bits and absurdist humor. (Rarr, we're team aqua, come to rob this museum! ... Alright everyone, line up to pay the admission fee.)
Basically pokemon is a pretty decent party based RPG. And really, it's pretty much the best game series out there aimed at children just getting into video games. If you look at most of the other stuff aimed at that early a demographic, a vast majority is just shovelware, made with the idea that kids don't know better, and that some parents will buy it anyway. For example, when our parents finally broke down and got us a gameboy after much pleading, they figured they would get their money's worth and get some games for my little sisters as well. One was a horse game in which you steered through a course, and jumped hurdles with 'a.' And that's pretty much it. The other was a series of abysmally awful minigames which wouldn't even constitute a facebook game today.