[vbuterin]

More precisely, Bitcoin mining is dominated by specialized hardware (ASICs), so non-specialized computers have almost zero impact or revenue potential in the Bitcoin network. Litecoin and Primecoin are less ASIC-friendly, so ordinary computers stand a better chance at actually finding a block because they don't have the massively overpowered competition.

[brownbat]

I guess the better question is why not Litecoin.

Or if you're going for something really obscure, why not Junkcoin?

I guess PrimeCoin is in 6th in capitalization... maybe it's in that sweet spot of not overly competitive but still capable of retaining some value: http://coinmarketcap.com/