Nothing is stopping it. There are already a bunch: Litecoin, Solidcoin, Ixcoin, PPCoin, Terracoin, (to a lesser extent) Devcoin, and (to an even lesser extent)

Nothing is stopping it. There are already a bunch: Litecoin, Solidcoin, Ixcoin, PPCoin, Terracoin, (to a lesser extent) Devcoin, and (to an even lesser extent) Namecoin. Seemingly the only thing stopping them from overtaking Bitcoin is Bitcoin's popularity.