I think the massive development of flash fiction as a format has been largely due to people reading on tablets, mobiles etc. I'm not complaining though. I started writing flash fiction by accident as I had a story that I couldn't pad out but wasn't long enough to send anywhere. I saw that Cafelit took very short fiction so I send the tale there and it went up on site. I've not looked back since. Many of my pieces on that site are very brief fairytales.
A short story has been described as a moment of change in a character's life. Well flash is half a moment! I use "incidents", not long enough in themselves to create a short story, to form a flash fiction tale instead. Occasionally I will take a flash story and expand it out to the standard 1500 to 2000 words length. I don't do this often as I'm focussing on creating new stories all the time but it is an option.
And most genres suit flash. Some of my pieces, coming up soon in my collection From Light to Dark and Back Again (Chapeltown Books), verge on horror. Others are crime stories. The majority are fairytales with bite. Flash is a wonderfully flexible format so whatever genre you write (bar epic I suppose!!!), you could use flash.