Because if there's only one choice that makes you competitive and/ or efficient it's not really a legitimate choice anymore. It would be like a talent tier that provides 1 clear DPS/ Healing/ Damage Mitigation increase and 2 flavor abilities that just look cool.... anyone who really cares about their performance will choose the talent that clearly increases your performance. It's no longer an "interesting" choice as Blizzard likes to say.how is choice in a video game bad?
Flying doesn't necessarily = better, but it's obviously much more efficient than traveling by ground, in which case the majority of people would use flying mounts simply because they can. Choosing between efficient and inefficient when all you're trying to do is get the job done isn't a choice anymore, the efficient option will always be chosen.