A simple answer would be the factors of supply and demand which makes the price of Bitcoin fluctuate like an oscillating object with no particularly set lower or upper limits. With that being said, we are nearing the maximum supply pool rapidly and with the ever increasing demand for Bitcoin, the price keeps rising till any external factors influence it and temporarily hinder its progress.
Many people nowadays are holding for profit and since we have a lot of demand coming in than the supply, we apparently are seeing some huge days for bitcoin. There is still so much likelihood of experiencing more of these as time goes on as this is pretty still small from what is upcoming when so many top players and countries start adopting its usage.
Bitcoin is taking over the financial world
I wonder when they will make it illegal, because the banks are but-hurt for loosing money.
You are wondering when they will make it illegal. If that is what you are waiting for, then it is best you kick yourself back to reality. There is nothing they have to hold against bitcoin and rather than declaring it illegal, which isn't, most countries are now seeing the aspect of regulating it and allowing it into the system since that is the best way they can control its usage through exchanges and then benefit from it as well.
