Your app icon is very dull and unattractive. The color is nice, but it doesn’t tell me anything about the game, or doesn’t make me want to click it to see more info. That is the first thing that people see, so they have to feel the urge to click it, otherwise they’ll just go to the next icon that is attractive and catches their attention. This may sound stupid, but it makes a difference. It’s not easy making a good catchy icon, and these days it’s much more important than it was 3 or 4 years ago. Note that I’m comparing your icon to the few good ones, not the massive amount of crap ones, which are far far worse than yours.
However, your biggest problem is probably that your whole game is just not very attractive in the first place. There are tons of visually attractive free games on the app stores, and unfortunately, that’s the most important thing for customers browsing the available apps. There’s nothing wrong with using low contrasting colors, but if you’re going to do that try to come up with a theme. For example, make all levels in shades of blue or something. Don’t just use random colors. Put some work into the graphic side of things. The animations are nice, but people can’t see those on screenshots, so you have to make them enjoyable on their own.
Sorry if this sounds harsh, but the online app stores are brutal nowadays, so unless you can pull a “Flappy Bird” you have to invest a lot more into the design of the game, and in making sure that the gameplay is solid. However, most people will never get to experience the gameplay if the game doesn’t catch their attention visually.
