Nor is there an article called "Newton's theory of gravity". But back to "theory of evolution", which certainly must have at one time had an article. (there was)
The original article created back in 2001 is simple and straightforward.
A theory of evolution is an attempt to scientifically explain how evolution occurs. There are countless theories of evolution, many of which are compatible with one another. Others, such as Lamarck's theory of inheritance of acquired characteristics, have been falsified.
In popular usage, "the" theory of evolution refers to the various theories proposed by Charles Darwin, notably the theory of common descent, and the theory of evolution by natural selection. It also refers to the modern Darwinian theory that combines evolutionary theory with the science of genetics.--------------------------------------
To the point, and exactly what you would expect when trying to find out what "theory of evolution" means.
Of course somebody may have been looking for "Darwin's theory of evolution", when they typed "theory of evolution.". But, there isn't an article for "Darwin's theory of evolution". It redirects to "Darwinism".
No, I am not making any of this up.