The gnomes make up 20% of their original race and were made playable the moment wow hit. They also together and have a district in Ironforge large enough to call a town, their racial leader, a city they're actively retaking(or retook) and irradiated populace that they're trying to cure.
The High Elves who are spread out, only make up 1% of the original population and have used what military force they have to support the Silver Covenant. They had no racial leader, they have no districts(unless you count Dalaran), no city to retake and no... population to cure(I mean they tried to cure their brethren with death a few times).
So unless the High Elves population was 20 times that of the gnomes, logically the Gnomes have a bigger population and more going for them.
Also Blizzard has also addressed the High Elves and Gnomes, saying that
In short, High Elves are not playable because there are very few true "High Elves" left -- far less so than Gnomes.No! More High Elves AND Blood Elves. Like I said before, their population doesn't need to be as big to be a playable race. They were made playable the moment they were added, for the sole purpose of being an allied race. And their a race completely made up of people willing and able to fight. You can literally say their population is 100 and it still works, because 100% of that is all fighters and blizzard doesn't have to worry about if they've put too many in the game because of previously established lore.And where do more Void Elves come from? Oh, that's right High Elves. If there's enough High Elves to maintain a Void Elf population then there's enough High Elves to support a High Elf playable race.
I'm sorry, but unless blizzard does away with logic, you're not getting High Elves. AND the fact that Blizzard is using High Elves to bolster the Void Elves means they're actively moving away from that ever being a possibility.