Because both the left and Islam have a common goal which is the destruction of Western civilization and the imposition of their worldview which in each instance is essentially authoritarian.
The Frankfurt School promulgated the so-called "critical theory" which has evolved into a reflex to discredit every value of our epistemology, our culture, our faith and, by extension, those atomic building blocks upon which a civil society is constructed. In other words, if you tear down the church, the family, the role of the father in the family, patriotism, the military which supports the nation, the educational system etc. you can so demoralize society and render it so insecure that you can then impose your world view.
Nathan Bedford's first Maxim of American politics, all politics in America is not local but ultimately racial, speaks to the idea that race is used not just to win elections or silence opponents but to render the whole country illegitimate.
So if we can't trust our religion, and we are damned as racists we are rendered utterly insecure and vulnerable and either Islam or the left will eagerly fill the vacuum and we will not like their methods. Ultimately these two competing worldviews must struggle until one or the other is exterminated but right now their interests are parallel.
