A less intelligent reason is:" the bad and insensible alliance shouldn't try to reclaim lordaeron because the forsaken are the poor poor poor people who lived there. And they chose their queen in a very democratic way. You would be heartless!!!" No. They are some (probably the majority) of the former lordaeron citizens and they have a SELF-PROCLAIMED queen (you can deny it forever, but in the end of the frozen throne she said "well, we have lordaeron, we betrayed everyone and I'm your queen ok?") who punishes generals in an unorthodox way and tests her plague upon his own people. As I said, a dictator. And then?
Then the forsaken are the rightful owners of lordaeron, but the alliance could try to reclaim (yes, reclaim, the alliance was in the past the alliance OF LORDAERON, does it tell you anything?) that wonderful plagued kingdom.
The problem: lorewise, the alliance CAN'T reclaim lordaeron. They have some adjacent regions in a complete state of shit which have to be rebuilt and they just lost a war (more precisly, they just won a war where they lost many many settlements). And lordaeron would bring now more losses than benefits.
Game wise, Lordaeron is the powerhub of the forsaken, one of the playble races, then is impossible in wow to see lordaeron in other hands.
And honestly, if the horde loses lordaeron then the alliance should lose northern kalimdor, otherwise bye bye gamebalance.