1. The largest population centers in Lordaeron were killed or raised into the Scourge during the 3rd War.
2. Lordaeron survivors in the Scarlet Crusade got mostly wiped out.
3. Lordaeron survivors in Theramore got wiped out.
4. Lordaeron survivors in the Argent Dawn got rolled into the Argent Crusade and remains neutral.
5. Lordaeron survivors in Stormwind are so few, they can't even form their own faction. Even the last remnants of Stromgarde could form a faction and actually do shit.
6. Forsaken raise hundreds of corpses every day at graveyards. It's been almost 2 years since the beginning of Cata. By now, they greatly outnumber any living Lordaeron survivors.