No worries, it’s a good question 
The difference between the high elves and blood elves is a political and historical difference. High elf refers to the elves who stayed on the alliance (such as those in the silver covenant) and blood elf refers to those who followed Kael’thas and wound up joining the horde.
For people like me who want High elves, the main difference is about character history. I’d like to create a Thalassian elf who is alliance aligned and stayed with the alliance the whole time. I personally like the idea because I was introduced to warcraft in Warcraft 3 where high elves were one of the main races on the alliance, and I thought they were cool
Unfortunately for people like me, blood elves and void elves don’t really do the trick. Rolling a blood elf makes you horde aligned (kinda hard to RP that you’re on the alliance in that case) and void elves impose a lot of character history on your toon, plus they got all this shadowy stuff going on and can’t be paladins.
I also see a problem with using Void elves as the stand-in for high elves since it kind of detracts from their unique identity too.
I understand that for a lot of people, this is not a big deal, but for me and many others, we think adding playable high elves would be an easy thing Blizzard could do to help make that character fantasy come alive.