Why have humans remained one species (the same)?

Because there is no reason for change.

Explanation:

Before we evolved into what we are today, climate was constantly changing, land was changing, creatures were changing. So there was need to change from species to species to be able to adapt to a niche and be better at surviving. Nowadays, we live lives of comfort and there is no need for our bodily chemistry to change as humans are perfectly adept at living in the world we have created. However, we are still changing to better fit it, we will likely never have to evolve into a whole new species.