The blood elves know very little about the troubles of present-day kalimdor and the horde's issues on kalimdor with various factions and species.
Putting a blood elf to be a warchief is like putting a night elf to rule the human and dwarven kingdoms - it is a completely foreign land, with completely foreign cultures and various problems and needs and even completely different types of society structure and politics, not to mention the obviously different history.
A warchief of the horde will (probably) never be a forsaken or a blood elf;
They are just too distant (both geographically and culture/awareness wise) to be efficient and successful leaders of the horde.
They are also vastly more interested in their own lands and their own people - undeads and belves never gave a damn about what happens to taurens or trolls or even orcs (let alone goblins).
A real life example would be if you put the ruler of Pakistan to be the leader of the United Nations - just massive differences in pretty much every conceivable way would prevent any efficient work from being done.