There are a couple of good reasons- once you sell, it isn't as easy to genre-hop around at your leisure, especially if you're hoping to stay at the same publishing house/imprint. And getting a couple of books under your belt that are the same genre can definitely help strengthen your writing skills and help you find your voice.
Still, though. DOESN'T THE IDEA SOUND REALLY FUN SOMETIMES!?
Since I'm unpublished, making the change was simply something that I had to get used to the idea of myself. After writing three contemporaries, I started to wonder why there wasn't more straight up, classic horror in YA. And while I love contemporary (and always will!), I realized that I was pretty passionate about trying on my horror gloves for awhile.
"This'll be easy," I thought to myself. "I've read a lot of horror. I've loved horror movies since I was a wee thing. LET'S DO THIS."
Once you start, something becomes pretty clear almost immediately: writing a genre you aren't used to can feel reeeaaallly uncomfortable and unnatural at first. If you're trying out dystopian, maybe you're finding that your world-building skills need some work. Or if you're dabbling in contemporary, you may find it a bit more difficult to concentrate on so much emotion when you're used to being able to throw in a magical vampire-bunny for excitement. For me with horror, writing any scenes that were non-straight-up-scary just felt off.
That's when that sparkly new inspiration and eager drive can falter, because you start to freak out and go all "I CAN'T DO THIS I NEED TO JUST STICK TO WHAT I KNOW WELL" and scurry back to your old back-up book in your previous genre with your tail between your legs.
Ah, writers. We're so silly sometimes.
Of course it's going to be insanely awkward at first. The perfectionist inside of you isn't going to like it right at the get-go. You're going to have to force it for awhile, maybe even going through thousands of words before you feel the 'click.' And even then, remember the oh-so-important-for-your-sanity rule, ALL FIRST DRAFTS SUCK. You're doing something new and scary, but the more you write, the easier it will get, and before long you'll have owned that obstacle right in the face.
So I say, write that book about the alien that works in a diner in Montana! It can only open more doors to you, both career-wise and creatively. Expanding your horizons can really help you grow as an artist, and just in case you needed to hear it...YOU CAN DO EEEET!