A: Because all crypto-currencies to-date are speculative "investments" (gambles), not currencies.
The demographics of crypto-currencies are currently speculative gamblers, not currency users. Thus there is always a demand for a new speculation which can be purchased earlier and cheaper. If someone ever creates a currency, then the users will demand there only be one, because having one unit-of-account means currency users aren't stressed with exchange volatility. Exchange volatility actually decreases currency use, because users have an incentive to not spend when the price is lower than their entry price, and not sell when the price is going higher.
Btw, the reason we are moving to a global currency is because currency users are beginning to transact globally more often, e.g. ordering online from Alibaba or DHgate, migrant workers remittances, etc..
I'm sure it's been discussed already, but do you know of anywhere on this forum (Maybe one of your Martin links talks about it? Come on man! I've got a job to look for today, too! ) I can look to that discusses exchange volatility in cryptocurrencies vs. exchange volatility in fiat? I'm trying to understand the correlation between your statement and the observation that multiple forms of fiat exist.
I can think of many things outside of exchange volatility as to why that is, but I'm sure someone's discussed this before?
Admittedly, your point about the move toward a global currency because of increasing global transactions has merit, but maybe more than just that can help me understand the proliferation of hundred's of different fiat currencies to date, in the face of exchange volatility?