I think it was because Nixon wasn't "one of them". They were/are Ivy League, from the rich left wing. Nixon came from poverty, and was educated in all the "wrong" places, Whittier College and Duke law school. So common, don't you know. Nixon also clawed his way up from nothing -- sometimes by playing things pretty close to the vest. It figures, since he financed his first politcal run in California, so it is said, from poker winnings in the Navy.
Nixon was the original RINO.
He gave liberals OSHA and the EPA. The leftists still hate him and use him as a sign of gratitude even though the man is dead.
So much for reaching across the aisle. McCain and Nixon will have an interesting conversation in Heaven.