Lets talk about that Lore. When the orcs started destroying all the human kingdoms during the second war, the response from the elves was “Meh … not our problem.” They sent only the absolute bare minimum support, barely a token force. Then the trolls joined up with the Horde and started burning Quel’thalas to the ground.
Suddenly the elves are up in arms, demanding aid from the human’s who they’d just decided to ignore. This set the stage for the “betrayal”, as the Elves proved themselves to be fair weather friends at best, content to let the rest of the Alliance die so long as there was no threat to Quel’thalas, but demanding 100% of the focus as soon as there is.
Fast forward to the scourge. Arthas and his forces are fighting off the undead, with again, minimal token aid from the Elves. After his expedition and his return as a Death Knight, here is the condition of the Alliance: Dalaran is besieged, Stromgarde is desperately holding off its own borders; Alterac died in the second war, Stormwind still recovering from the first war; Gilneas walled itself away; Kul Tiras isolated at sea (not to mention a primarily naval force which would be less than helpful for an inland invasion); Ironforge half a continent away; Lordaeron, utterly destroyed. THEN, he turned his attention to the Elves.
So this great betrayal in their darkest hour … who was supposed to come save them? The Elves had already let all their “allies” die first. Just like they always did.