For one, because they do. The website which contains the claims of evidence has several small internal inconsistencies and notes several cases where the prosecution witnesses do not confirm identical beliefs.

One of the damning things about the DNA evidence is that the experts claim odds of "One in a trillion or two" and "Ten billion to one" that the DNA matches a random person. That requires that the odds of a given person having an identical twin about which they are unaware be less than that, and/or that DNA from a given crime scene be expected to match from about one-half of a living human to a miniscule fraction of all humans, if all humans were tested.