RE: The dollar's time is coming...
There's the short answer and the longer answer
Short answer: because the Federal Reserve wouldn't allow it.
Longer answer:
These are just a few reasons that I think. Not trying to detract from how amazing Bitcoin is btw. I own Bitcoin, followed it since inception.
Policy reasons: Bitcoin is decentralized. The Fed and world banks will only accept a centralized crypto currency. They want control in a similar way the existing fiat currencies have some controls in place.
Financial reasons: Bitcoin is also a fiat currency, meaning it's not backed up by anything. I don't think backing a fiat with a fiat would ensure value.
Technical reasons: The Bitcoin network doesn't scale to global scale in terms of numbers of transactions. It's fairly limited in it's ability to process transactions.
Right now everything is on the dollar standard, IE global trade is overwhelmingly priced in dollars. Every country outside of the United States would like to see that changed. I would not be surprised if in the not too distant future we were on a bitcoin standard where fiat currencies where simply backed by satoshis.