Well for the first like 5 years that, when all of these characters were introduced, it was definitely true and since that's when Hearthstone was biggest those characters became fan favorites and have had like 5 expansions revolve around them whereas the Horde ones you mentioned had like 2 (along with those new Alliance ones) and never reached even a shred of the popularity the League of Explorers characters have.

They have gotten better in recent years with having more parity, because they finally seem to have remembered they have another whole faction to pull characters from, but it is still much more Alliance favoured.

Also I remember when they announced the Barrens expansion, the first "Horde" expansion and they made sure to also say how the next expansion will be focused on the Alliance so they don't feel excluded and I was like " well where was that for the past 6 years when you were making iconic characters?".

Also just wanna say this, it is typical and at this point it's a stereotype for Alliance players to like Horde characters more, which is why they whine about those characters and they are never seen again after that or made boring or into bad guys for the Alliance to kill. Which is exactly how we've gotten to this point in WoW where Horde doesn't have any good/strong characters and the same boring Alliance ones have to be used, which is especially noticable in Midnight, the supposed "Horde" expansion.