Because it defeats the point. There is no "coin" that will exponentially appreciate in value, making you a billionaire. There is also no centralisation, so you cannot be the owner of the payment system in the same way that Paypal or Visa are.
At best you can be the Linus Torvalds of your payment system non-profit. Which would be a good gig in itself. But that comes with lots of hard work, not the easy riches that you can do from some hype-pump-dump scam in less than 6 months.
To build on this comment, the purpose of a currency is to facilitate the transaction of value, not to store value.
If an instrument stores value, it's an asset, not currency. One can of course transact business with assets, but it's a lot more complicated because you have to agree to a valuation. And without a currency, how do you measure that?
Plus, the incentives for assets and currencies are opposed. Currency exists to be spent, but if an asset is appreciating, you'll want to hold it, not trade it.
Bitcoin was advertised as a currency but it's acting like an asset. (And it's regulated like an asset.)
If you create a new class of asset, you can buy in early, hold on, and get rich on appreciation.
But if you invent a new currency? It's like inventing the inch. Even if everyone uses it, that doesn't give you any more length of your own.
