Evolution would not be nearly as controversial but for the rabid athiests, men such as Richard Dawkins (and even Carl Sagan, in his own time), who jumped out of science into religion and philosophy--claiming evolution disproved God. Of course since science and the scientific method are based on unprovable philosophical/religious assumptions in themselves (as they must be), nothing can be pure science anyway. I get tired of the scientific elites who tell us all how much more they know about an infinitely large and intricate subject (beginnings) than the rest of us. To disrespect religious solutions at all such as by using "speggetti monster" ridicule too, only proves that people that argue like that have an axe to grind, and are not making scientific arguments anyway.
It's very possible to believe in evolution AND intelligent design....as a matter of fact if you're an evolutionist and you don't, then logically you reveal yourself as an atheist, agnostic or at least a deist. Please make that clear...so we know what drum you are beating. When science and scientists stick to the scientific realm, then you can expect philosophers and theologians to stay out of science. But the self righteous bleating BS of evolutionary scientists over ID, (making even the mention of it in a philosophy class in school a federal case...) tells me their's is a religious cause too, just like that of the creationists--only they hide their atheism behind "science."
What makes it controversial isn't just the rabids. The rabids make the conversation distasteful. The theory would be just as convoluted and controversial otherwise. Thus it's failure to take hold and it's waning influence.