Because unless you're equally successful "this guy was a tool" comes across as poorly concealed envy. If you're going to trash someone who's way more successful than you are, use specifics and acknowledge his positive characteristics as well.
Frankly, I'd prefer if our comments stood on their own merits (or lack thereof), but perhaps you've hit upon the next social media craze. Instead of simply using karma scores, we could multiply one's karma by the amount of currency tracked in their covestor account.