There are two of me. One writes adult and the other writes YA/NA. There's a simple reason why I write all three. I am an adult (and quite frankly, that stuff sells better digitally) and I loved high school and college. That was a great age for me, so I'm drawn to writing it.
Much to my children's sorrow, I will not write middle grade or chapter books. It's not anything weird like a lack of respect for the genre. I am HUGELY appreciative of those categories because those are the books that make lifelong readers. Problem is, I didn't come into my own until high school, and I don't look back on the earlier years with any fondness. If someone said to me "Hey, we're going to make you twelve again and you can live your whole life over!" My response would be "No thanks, I'd rather choke on my own panicked vomit and die." Plus, quite honestly, I don't have the right voice for it.
There are people who can bridge from picture books all the way to adult and do it all well, but they are few and far between. It always makes me shake my head when I see writer friends/acquaintances who seem to jump on whatever the "it" genre is.
"MG is hot? I'm going to write that!"
"NA is the new thing. Sign me up."
Now, anyone who knows me knows I'm all about trying new things and stretching as a writer, but if the reason you're doing it is because it's popular, you're doing it for the wrong reasons. It's like dating the captain of the football team in high school even though he's a misogynistic creep who never bothers to comb his hair or brush his teeth. Hey, he's still football captain. Hello...popular.
Hello... lame... stupid... ridiculous. (Pick an adjective.)
So is chasing the "it" genre. By all means, if it calls to your muse and fits your voice, have at it. But if you're just doing it because it's popular...
Do you know why I write (some) New Adult? Because I love writing YA, and some of my YA has always nudged into the college arena (I was told at the time it was unsellable). I also write adult, but I've been told I have a "youthful" adult voice. (Appropriate since Julie means youthful.) So that sweet spot for me? It's 16-25. Yes, I've written older characters, but that (even with my adult stuff) is where I like to sit. I'm just lucky enough that it crosses three genres. I have very little desire to branch out much further.