I guess the first best answer is: That's what sold and...that's what sells.
I wrote a novella on a whim. Ellora's Cave had a call for submissions: "Oh, Yum." Older woman, younger man. I'd been reading that to stretch one's "writing muscles", you should try something different - something you're not used to writing. Or, in my case, never wrote - read, but never wrote.
And so I wrote a novella that after almost a year, was rejected by EC.
At NJRW conference last October, I had an appointment an editor from The Wild Rose Press, a small press/epublishing company that was growing by leaps and bounds. So I pitched it and the editor said she wanted to see it. I sent it and a few months later, TWRP bought it - much to my surprise. I had written it on a whim.
Now, I'm doing edits, have a pen name so I can write under other genres, and working on promo items for RWA National in July. I have a contract! I will have an e-book soon.
Am I excited?
No one is more surprised than I am.