Because it's more stressful on the healers, and you are more likely to have aggro pulled without the tools to pull it back so easily. With prot, you effectively have 2 taunts and generate MUCH more threat.
a GOOD prot warrior will outperform a GOOD fury warrior for tanking 100% of the time.
Your job as a tank isn't to "top the meters" it's to take AS LITTLE damage as possible while holding threat. No healer likes a tank who takes a crap ton of damage, even if it's theoretically possible... i mean its possible for a shaman or rogue to tank too but you don't WANT it.
I can't even count the healers who see i'm really prot and fall to their knees in joy at the site of it. If only i had a silver for every time i heard a healer complain about fury warriors... i'd have 3 epic mounts