The reason why angels are not included is that Darwin’s theory is based on a purely materialistic philosophy (scientific naturalism). Ironically, another reason is actually religious. Most traditional religions believe that God created angels as a separate species from the human race.
Two things are necessary to include angels into a plausible scientific theory of evolution. First, angels would have to evolve from the human race. And second, evolution would have to show a trajectory toward a non-material realm.
The physical evidence shows that the trajectory of evolution is toward increased consciousness and intelligence. So whatever niche a successful species exploits along the way, increased intelligence has been the tangible result.
Humankind enters the evolutionary scene by exploiting a most unique niche, which perfectly supports and nourishes increased intelligence. Rather than requiring only physical food, the human race seeks mental food—information.
So, if we logically look at the entire unfolding of evolutionary process to date, humankind is an obvious means by which the trajectory of increased consciousness and intelligence can be maintained. So, such a scientific model has explanatory powers. But, does it have predictive powers?
Where does increased consciousness and intelligence go from here? The answer, even in Darwinian terms, would be something that would increase the chances of continued success for the human species. Some people believe that humans are headed for destruction—destroying themselves and the planet. With all the problems facing the world of humanity it seems that wisdom is needed. Wisdom most certainly is a more evolved state of intelligence and consciousness.
Here is where religion lawfully enters into the equation. Religion offers values for the human heart and mind to live by. Wisdom is the result of recognizing and choosing the best values, which treats of fairness, justice and has as its goal the exaltation of human society. It seems crucial to the successful trajectory of human intelligence that that as a species we must become more angelic.
Science definitely overlooks the importance of values in its current theory of evolution. And, you might challenge such a theistic model because angels, if they exist, are non-physical beings, living in a realm beyond space and time.
Some scientists (not all) believe that the human mind operates beyond the constraints of time and space. If wisdom represents a better organization of the human intellect, then this means bio-complexity (higher order structure) can evolve beyond its involvement with time and space. Which means that we can maintain our individual integrity and personhood after the death of the physical body.
Scientist/theologian Emanuel Swedenborg claimed that the purpose of creation was so that the Lord God could create a heaven from the human race. I predict that in this century scientists will move towards a more theistic model of evolution. I have written a book, Proving God, to help point the way to such a new theory of reality.