Zarhym says that it's just part of the game's evolution -- originally, the dreadsteed quests were part of the endgame, and just like the Hunter Rhok'delar quests, were designed to show that players had reached the pinnacle of the class. But nowadays, level 60 isn't what it used to be, and the best way to do those quests is to have a higher level character tag along with you. That's not how Blizzard designed it (and it definitely doesn't fit within the "lore" of the quest), so they ditched it.
That questline is still in, just not required, but some questlines and rewards are obviously removed from the game completely -- their achievement no longer represents an equivalent challenge, so Blizzard decides to take them out. And that really seems the best way to do it -- though it's always a shame when today's players can't experience the quests the same way veteran players did back in the day, the alternative would be to have them play through content that makes no sense, and no one wants that. As Zarhym says, there will always be new challenges to take on.