The first stories I wrote were for competitions and limited to 3,500 words. For many years I thought I would not be able to get above 5,000 words.
When I wrote Chronicles of Mark Johnson for some friends, I wrote a series of short stories, it was only later they became the award winning novella.
I had a friend who wrote novels and I was worried about spending such a lot of time on one project, only to see it die painfully. What damage would that death do to my morale?
Some authors like to write a full length novel and don't worry about whether their work is accepted. By writing short stories, I can see how my work is accepted an judge if I need to change things.
There is also the time Vs income concept to account for. Would investing all that time be worth it?
A short story can take from two hours to four for me to write -- depending on length -- and while not bringing in big chunks of money, the story brings something in.