why are random selection and intelligent design mutually exclusive.
It's not entirely clear since advocates of "intelligent design" steadfastly refuse to say or postulate anything about how (or when, or where, or by what/whom) acts of "design" are actually implemented. But they do assert (baldly) that acts of design are non-natural. IOW they are "inferred" when we can eliminate (don't ask me how) "natural causes".
Yeah, I know it's a mess, and intentionally shielding your mechanism from scrutiny is completely unscientific... But don't complain to me. This is what ID'ers say that ID is.
why are random selection and intelligent design mutually exclusive.
They're not, in principle; one could easily imagine a cosmos in which an intelligent designer imposes various levels of order on an underlying process that's undergoing random variation. Indeed, Plato's Timaeus describes such a cosmos—the Demiurge whips the pluripotent and randomly shaking Receptacle into shape using mathematics and the eternal Forms.
But the project of modern empirical science dispenses with final causes, intentions, in favor of lawful but impersonal energetic interactions. The evident successes of science bespeak the wisdom of this project.