That brings up an interesting point actually, could it be that Wario just plain lacks in the "Kid appeal" factor?
Most people probably learnt of Wario trought Marios games, but let's compare him to the other 7 of the "Mario Big 8" :
These are the first characters that come to most peoples minds when you ask them about Mario, but what makes each of thrm interesting from a kids perspective:
-Mario: Iconic and instandly reconicable.
-Luigi: Lovable coward and buttmonkey, almost as iconic as his bro.
-Peach: Pretty Princess.
-Toad & Yoshi: Adorable and friendly.
-Donkey Kong: Doopy friendly (outside of Mario VS DK) cartoon animal.
I don't really know how kids think of Bowser, althought kids are more likely to gravitate towards the heroes rather than the villains.
Wario falls in to a similar nieche as Bowser, while not a Villain per say, he isnt really a good guy either and most kids probably don't like him for beeing more mean than others.
His games also have a similar issue, that's not to say that kids can't enjoy the wacky nature of them, heck, I was 4 years old when my older sister introduced me to Wario's Woods and the first 2 Land games, but I remember playing Land 4 during breaks in elementary school and some of my classmates beeing scared of Spoiled Rotten when his health goes low and that's only the first Boss! God forbit if they knew about Rudy or Land 2's very final level...