Because the wildly libertarian China that has state firewall and can cut off the access to the block chain completely will just fall in love with something that could for any reason diminish its ability to have iron grip control over its economy and currency.

If Bitcoin is going to be not fringe it will need wide state support of at least 51% of the world GDP. There is not a single economic bloc that could endorse the loss of the ability to manipulate its own currency.

It is an awesome real time payment system, but crappy as a currency.