An Unpredictable Question about Bitcoin "?"

As you are here so i wish all of you having better idea about the crypto world. Still i have been present few lines because of my intensity. Technology has been developed day by day.A net of Blockchain and cryptocurrencies cover all the world.Decentralized mechanism establish it more interesting. So many cryptocurrencies are presented in the market. System make the bitcoin as the mother currency.
So many experties think that Blockchain is the future and next revolution.
Maintaining and following all the ideas there should be question, why bitcoin value is decrease day by day.

December 2017 was the golden time bitcoin and founds it's highest market value.After that it has been decreasing.According to coinmarketcap.com on today its value is $6358.68.
So the question is why it has been decrease ??
Those who are having clear idea about it please share.
I wish a positive reply and support.