Most likely the 20-80 rule is in effect. The top 20% earns 80% of the revenue while the remaining 80% will have to share the remaining 20% of the revenue.
Some of it is luck, but the games that are successful tends to have good marketing.
Marketing does not mean advertisements though. Rather it means market segmentation, find your niche and appeal to it. Build and interact with the community to generate hype. Make a game that is good enough.
The better mousetrap fallacy is in full effect, you can make the best game there is, but if no one knows about it what's the point? But shotgun advertisement is a very bad way to build awareness. You want to target your marketing efforts to your core audience. If your game can have a playable demo it should have one.