For YA fiction to be popular it needs to have characters and themes that will relate to young adults and their interests. That is all, and maybe also more modern language. This doesn't necessarily make it unpopular with adults, nor does it make it any less worthy as fiction. Adults might be slightly less sympathetic to a very young protagonist, however that would be more to do with the quality of the writing than the age. I can think of several adult books where the adult protagonist was either very naive or unworldly, or else had learning difficulties, and yet they still worked for me.
What I wanted to say, however, was the real reason that I read the 'Harry Potter' series, 'His Dark Materials' series, the 'Noughts & Crosses' series and the 'Hunger Games' series, was not because I thought it would be great fiction (though I would recommend them all) but (at least in the first instance) was simply so I was aware of what my kids were reading. There is so much talk of monitoring what your children are looking at online and the games they are playing, but nothing about what they are reading. And it is good to be able to discuss books with your children.