There's a fundamental truth in all this that I think devs are willing to ignore, because it's a hard pill to swallow. Let's say:

The game is good.

The marketing was great.

...AND sales were abysmal. What's the reason? The reason sales were abysmal is because... well, the sales were abysmal.

Sometimes you're a meme, and sometimes you're nothing. That's just the name of the game in our globalized marketplace that is hypersaturated in every way by every kind of media. Sometimes you can get 10 diehard fans to recommend your game to 40 friends, one of whom writes for a major gaming review site, which gives your game high reviews on the day that a major storm hit half of the country, so everybody is inside reading that review, so lots of people buy your game and you make bank.

All you need are a few stories about how interesting the characters are or how fun the mechanic is, and that's it. You just need 2 days where people care about you, mention you over the water cooler, or post a joke about your content that gets to the front page. Those 2 days can change everything forever.

OR you can do exactly the same thing, but one of those people who would have loved your game came down with the flu, who would have otherwise pulled his entire social circle into your game, and it was a bright and sunny day when your review came out, so no one read it, and then it immediately got buried under more and more media; particularly, even newer, even shinier games (even if just by 1 day).

A hyperbolic example, maybe, but not entirely divorced from reality. "Success" in an industry that's about recognition relies on an incalculable amount of luck. Failed actors who got critical acclaim for one role and somehow didn't get any further don't do post mortems. Musicians who never got called back by the London Symphony Orchestra don't do post mortems.

BUT indie game devs do post mortems, so it kind of seems like maybe they can figure out, in retrospect, what they did wrong.

Unfortunately, the answer may very well be "nothing". You did nothing wrong, but the stars didn't align. Flappy Birds made more than your game did, and it's not because Flappy Birds is better.

What separates mainstream games from indie games is that the mainstream has the inertia and the spending power to dictate that their games are necessary. They get to say "new Call of Duty soon, you guys all want it, right? Of course you do."

Indie games just get to say "Hi, I'm here, and I hope you like me!". But where it goes from there isn't always up to the devs, isn't always up to marketing, and isn't always about someone doing the right or doing the wrong thing. A butterfly flapped its wings, and now we all know who Edmund McMillen is. We all know who freaking Notch is, the man who grew a mountain fortress out of a sand castle.

Working hard and making a great game is just the bare minimum now. The rest may as well be up to the Gods.