It was logically safer to have more go to stormwind than hyjal. Hyjal isnt that far from Orgrimmar and during the fourth war one of the Horde’s imperatives was taking and maintaining dominance of Kalimdor. Had all the night elves gone to Hyjal the Horde would have marched on Hyjal.
Remember Sylvanas had the Horde absolutely convinced that any major alliance presence or settlement in Kalimdor posed an immediate existential threat to them.