Because Blood Elves and High Elves are the same Model. It makes zero sense for one side to get a brand new model and not the other. High Elves have not had a significant change to justify it. Any new model High Elves get would have to apply to Blood Elves and Void Elves. It doesn't make sense otherwise.
Why does this matter for factions? Because since Humans are the parent Race on the Alliance Kul Tirans are free to have a new model. If Blood Elves had originated on the Alliance, then High Elves could have a new model but since they originated on the Horde and nothing has been depicted to show a significant change/reason for High Elves to have a new model then it makes zero sense for them to get one.
Half Elves however make all the sense in the world to get a new model because they are Half Human and Half High Elf and can continue the story of High Elves without stepping on the toes of Models.
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Makes sense to me.