Because demographics are against the Republican Party. They seem to be actively attempting to purge everyone from the party that isn't a white christian, and with demographic trends being what they are, that is a recipe for disaster. "Small town America" has been paved over by suburban housing developers and populated with emigrants from around the country. The Atlantic coast is trending D, potentially leaving the Republicans with only the sparse electoral votes of Appalachia and the Plains states.
Exit polling overestimates minority turnout and than to compensate they overestimate minority Republican support.
Bush did not win 44% of the Hispanic vote, he won much less.
IMO, the only way to swing Asians is if affirmative action and racial quotas become a topic once again.