Religious belief is not about evidence. It is that which is believed to exist in lieu of evidence. It is belief that derives from the desire of contrafactuals alone.
And so the problem with an atheist saying something like, "I'll accept that atheism is wrong if I were given clear evidence for the existence of a deity" is that, since religious claims are believed through religious faith -- belief without evidence, belief that is not contingent on what can be demonstrated -- any evidence that does come up would support the claim, but not the religion. The claim would have been wrested from the claws of religion and brought into the realm of science and reason, but the religion itself would not have scored any points, since it does not see evidence as, you know, evidence.
To religious beliefs, evidence is not seen as a primary reason to believe a claim (as in science), but rather as a form of apologetic to quell your intellectual concerns and curiosity, and a secondary reason for belief in the religion right after faith itself.