So I sold a dozen short stories, convinced an agent I had a novel, which I didn't, and when she wanted to see an outline, wrote the novel real quick so I could do an outline. She couldn't sell it, but she liked it, and was thus ready to market my second book, which did sell.
To somebody who had no idea I had written any short fiction, by the by. So much for needing credits.
Wasn't long before that entry to publication changed. Short fiction markets dwindled, and pretty soon, there were more novels being published than short stories, leastways in F&SF. Just as likely to sell a book as a short, and for a lot more money.
True, it takes a lot longer to write a book than a short story. More words, although I have to say, I think the novel form is much easier than the shorter ones. You have room to screw around in a novel, go down some interesting side roads, and as long as you loop back later, no problem. With a three- or four-thousand-word short story, every line has to move the piece forward, so it has to be lean and mean. You can't get far from the main point, and that's usually narrowed to either plot or setting or character, and no space to detail the others. Shorter it is, the harder it is to pull off.
What with the rise of the internet and epub, there is now more of a market than there was, say, twenty years ago, but those length constraints still make it tricky. Much more productive to write a couple chapters of the novel than a self-contained story, and it pays better. If you get a nickel a word for a 5000-word piece, that's not bad. Even a so-so advance for a 75,000 word novel is twice that.
So, better money, easier to write, that's why I prefer doing books to short fiction; however, that said ...
Now and then, an idea sprouts and it doesn't need (nor will it justify) the long form to tell it. When that happens–and the reasons are varied–then I will sometimes crank it out. Usually it is when I am too busy to spare the time. Often it comes externally–somebody is opening another market I find interesting. Usually, there is enough fire behind the idea that it happens at a sitting, or maybe two. Clean it up, send it off, go back about my business.
I've done some originals this way that went straight to epub. The Roy the Demon collection, a handful of others. Charge 99-cents for them, and won't make any money, but the itch gets scratched.
In the last few months, a trio of markets arose unexpectedly, and the result wound up being four short stories: A fantasy, a high-tech fairy tale; a military SF; and an SF short in somebody else's universe. Two them went to one market.
The jury is still out on three of the four, and I may not sell any of those, but even if I don't, I don't regret writing them. If they get kicked back, I'll stick 'em up as epubs. If they do sell, I will be pleased, even though in one case I won't make much, and in another, I'll lose money on the deal. (I contributed to a funding campaign for the anthology because I thought it was a cool idea, and if they buy the piece, it won't cover my contribution. Still, that will be the most fun of the bunch for me, because I really, really wanted to write that one ...)
I'll let you know how it goes ...