Operating systems' success is fundamentally based on their popularity. Popularity resembles an exponential distribution, therefore there can only be a small number of popular ones with a long long tail of less and less popular ones. Here, popularity is a positive feedback loop, where the more popular it is the more developers target it, and the more it's targeted the more it's developed internally, which makes it better and therefore more popular.

That doesn't mean it's useless to develop them. If you set your sights low (playing around, learning, academic) you can still benefit from them without them being a huge success by popularity standards.