That might be because You joined a discussion that’s been going on for years across multiple threads. It’s quite unreasonable to expect people to repeat themselves for every person that comes late to the party.
But to satisfy Your curiosity, here’s one of the reasons:
By adding a race called High Elves to the game Blizzard would not only please players like You, but also upset many Blood Elf players.
Blood Elves are a direct continuation of high elven society: they are the same people (literally - any adult Blood Elf that was introduced in TBC had to have been alive before the third war), they inhabit the same land, speak the same language, uphold same beliefs and customs (worship of sun, connection to Sunwell), have the same culture (reliance on magic, same architecture, etc).
By introducing a race called High Elves Blizzard would essentially invalidate that continuation and appoint this new race (used here as gameplay term) as “rightful heirs” to over 6000 years of Thalassian history and culture.
Adding “real, true High Elves” to the game doesn’t only give some people what they want - it actively takes away from others, robbing them of their heritage and making them second class Thalassians.
This has already been said multiple times, even in this thread: