More than anything, stories get popular because their authors are already popular. Case in point: The Cuckoo's Calling
receiving a surge in popularity once it was revealed that Robert Galbraith was in fact J.K. Rowling.
How to be a popular author, and how to write a popular story, are similar but different. Let's assume we're talking about the latter and that you're not a popular author.
First, as Vimbert said, front matter is everything. The metric of popularity you're actually using here is views. I doesn't even matter what's in the story, because once they've opened it, you've got their view. The actual content of the story itself will have an impact on recommendations and whether or not that particular reader will follow you or read another one of your stories, but in isolation it's irrelevant to that view. What matters is you have an eye-catching image with a bombastic or intriguing synopsis.
Write enough of these stories, as Azu said, and eventually you'll build up a substantive number of followers, at which point all of your stories are guaranteed a view from each follower plus the eye balls you'll get from the feature box.
That said, to get a story to be popular to the same degree as Fallout: Equestria
or My Little Dashie
, the content is important. It has to strike a chord within your audience strong enough that they have to share this feeling with other people. A throwaway comedy with a snappy cover image will never get more than perhaps ten-thousand views and will be forgotten a week later.
Of course, there are a whole bunch of other factors as well. Easily digestible stories will obviously win over complex ones. Timing is a huge factor. What is it that the market is lacking right now? This very easily explains the popularity of books like 50 Shades of Grey
. (Of course, the Internet is not particularly good at understanding the mindset of mid-'40s stay-at-home housewives, so they'll say the reason is obviously pure luck.) Similarly, Fallout: Equestria
was the first epic-length story in the fandom and owes a lot of its popularity to timing (and the added exposure that its permanent position on the EqD sidebar provided). It became culturally relevant to the point that when a new chapter was released, most people would read it that very day. Likewise for Past Sins
But I'd rather change the question at that point from "How do I make something