Bitcoin exemplifies some of the problems of private money. Its value is uncertain, its legal status is unclear and it could easily become valueless if users lose faith. Besides, if bitcoin ever really started to take off, governments would either ban it or take over the system.
Uncertain = maybe this means volatile.
Legal status = not really needed or required, but would be nice if "accepted" by governments, like Fincen etc.
Ban it = how do you ban the internet? Try banning torrents.
Take over the system = well, they can certainly do that, but at what cost to them?
But I suspect another important factor is political: bitcoin appeals because governments are not fully living up to the responsibility that comes with state-sponsored money. Bitcoin, or something like it, will thrive until the authorities do better.
And authorities can only do better by creating a decentralized coin that has initial funding from the government.