yeah, hashcash is a very neat idea! but had problems like: how do you determine the threshold for the amount of work to be proven? there are values that makes it too expensive for a well intentioned human, and not enough for a bad intentioned spammer... moreover, it would induce economies of scale like it happens in bitcoin mining (spammers would invest in ASICs etc.) Signatures, on the other hand, allow to cheaply leverage other forms of "capital" (digital identity, github activity)