Bitcoin will not be able replace fiat for now because of several factors. Among them are unstable bitcoin exchange rates and international laws protecting bitcoins dont' yet.
Because units are very small in bitcoin, ie satoshi. Too much trouble if you have to shop in a store with a lot of decimal.
Too much decimal is only a tiny problem. It's mainly about the decentralization factor that banks and governments can't accept it as main currency. It's all about power and politics. But let us not dwell into things like that. The good thing is bitcoin is well accepted in most countries and this matters most.