Very simply - if I as a client have block X, which I know has been seen by MtGox, TradeHill, and a dozen mining pools and trusted parties... I can quickly and confidently reject block Y which purports to replace block X with a higher proof of work.
As long as the blocks contain adequate proof of work, they are all eligible to be accepted. It's not a beauty contest where the one with the highest proof of work wins. The reasoning works if we replace "block" with "blockchain" on the basis that longer blockchains are preferred.
Suppose the bitcoin network divides into two roughly equal portions in terms of hashing power and trusted parties but without any attack. Life goes on for a while at roughly half the block rate and with half the trusted signatures missing. When the network reconnects what happens to the losing chain and the trusted parties' signatures of the losing chain's block hashes?
ByteCoin