I have explained in another thread of mine why Bitcoin is following unrealistic uptrend. Though there are multiple reasons for this. But I can give you one single answer is because Bitcoin is the first product of a new technology and every new technology follows exponential growth which means it doubles its value at constant interval of time.
So initially it will appear as if nothing is happening, but after a break even period of time it will explode as if it is going to infinite, but it will not be infinite in value. The same thing happened to Bitcoin. During the first few years of testing and trials few people kept on saying that it will die and it will be zero etc, but it has been doubling its value in small amounts in constant interval of time, not exactly but approximately.But since the market capital was small and hence, no one was paying attention to it, but now since the value has increased too high people get fearful, but the pattern remains the same.
The fundamental reasons behind Bitcoin price explosion is because Bitcoin is the FIRST, DECENTRALISED, CRYPTOCURRENCY of the world based on the world revolutionary technology called BLOCKCHAIN technology of having FIXED supply of 21 million Bitcoins in total. These are the 5 fundamental reasons in bold letters.
There are 3 practical reasons behind Bitcoin price increase till now and in future:
1.ICO(Initial coin offering) funding using Bitcoin and number of ICOs will increase rapidly in coming years and hence, Bitcoin usage will be increased as well
2.Institutional investors using Bitcoin as a store of value as a replacement to gold, stocks, bonds, real estate, currencies etc.So all the trillions of USD from these markets will slowly start flowing into Bitcoin and of course, other cryptos as well. But Bitcoin will definitely be the first option due to it's high value and small limited supply of 16 MILLION BTC total.
3.Sudden country wide adoption, usage and conversion of a specific country's currency to Bitcoin due to financial insecurity. Whenever one country joins Bitcoin, one big price move happens. What a bunch of hundred people or thousand people will do may create a temporary impact on Bitcoin, but will have no impact at all on the overall Bitcoin price in long run.
By the way, the funny thing which I can see in future is when fiat currencies will be useless and will not be accepted for daily transactions and it is not so far and it is a couple of years may be between 5 to 10 years and may even happen much earlier. I mean now few vendors and companies are accepting cryptocurrencies and then, after few years all vendors will start rejecting fiat currencies and will accept only cryptocurrencies.