To theorize

[Syegfryed]

Why don't the Blod elf ask the Horde for help?

Are you asking for an ingame reason for a decision the design/lore team made?

They want the shit elf tribes to unite, so they will only put elves, they will either ignore the rest of the horde/alliance forces, diminish then or just say they are depleted due to the recent conflicts, they can always find a reason

[Soon-TM]

That was actually the question, but my Ingles failed me.
Why don't the Blod elf ask the Horde for help?

Because that would imply writing the Horde as something other than pants on head retarded evil guys or docile Alliance doormats. Something that is clearly beyond the ability of the current writers.

[Feanoro]

Why don't the Blod elf ask the Horde for help?

Because they have always viewed them as useful dumb beasts and meat shields, and since the Horde keeps proving them right by going full WAAAAGH when some psycho says "For the Horde!", they have no real reason to change that view.

Blame the writers for failing to make the Horde interesting or have anything beyond "ORC SMASH!"