I'm a co-founder at a startup called https://appstorerankings.net and I talk to app developers about this all day. The biggest problem with smaller app developers is that they do not spend enough time marketing their app or figuring out how their user-base will find them. The standard development path is to spend hundreds of hours on developing an app, then as an afterthought spend a couple hundred bucks on an app icon, release the app hoping that it will do well. Hoping!

There is a reason why companies like Flurry, etc with incentivized app downloads are doing well -- it's because they push apps to a point where they get discovered via organic search. If you really are going to want to launch an app, you'd better hope you have one of these two things: a $X0,000 marketing budget on incentivized downloads or the ability and willingness to slowly grow your app organically via long tail keywords, app store optimization, hounding app review sites to cover you, etc.

Heck, many apps' developers don't put a lot of effort in their screenshots, keywords, etc. There are thousands upon thousands of 'zombie' apps that are otherwise great but sit undiscovered simply because the developers thought that apps could market themselves.