Obviously you haven't played WC3...
So the Alliance and Forsaken team up to take back Lordaeron which they agree will go to the Alliance when all is done. Ofcourse by the end Sylvanas doesn't hold her word and kills the Human survivors and then appoints a Dreadlord as her second in command of which seemed to be quite high up in the Burning Legion and is part of a race know for their treachery.
So after killing Lordaeron's survivors and appointing a Dreadlords as their second in command, why should the Alliance trust them?
It's not racism it's going by what they've the moment they had free will.
Then in modern WoW we have their current activities of plague, necromancy and attempts to slaughter everybody in their path without mercy meaning that their current relations can only be worse again because of the Forsaken.