Because it's divisible by up to 8 digits. Therefore all 7 billion people on the planet could at least own a few fractions of a Bitcoin. Then there are Litecoins, of which there can be four times as many, and there are others out there.
I'm not sayin' Bitcoin is the end all. However it does seem to offer much greater utility, economy, and freedom of action far above what we are currently using as a medium of exchange.
I don’t disagree with the concept of a digital currency, but I have some doubts about the bitcoin is all. It’ll be a lesson for us whatever happens.