Scarce Resource
One of the main reasons for Bitcoin's value is that the supply of Bitcoin is limited.  21 million bitcoins will be supplied over 18 million bitcoins have already been mined.  Because the value of Bitcoin is determined by the ratio of supply and demand.  Day by day the demand for BTC is increasing and the number of Bitcoin holders is increasing due to the growing population and the price is likely to increase in the future.  Experts believe that competition between holders will increase the value of BTC in the future when the supply of Bitcoin reaches 21 million while mining Bitcoin will not.
Decentralised Network
Another important reason for BTC's value is that BTC is a fully decentralized networking system.  Centralized network systems could be controlled by governments like traditional currencies or controlled by one or more institutions, individuals and groups.  But since Bitcoin is a completely decentralized system, it cannot be controlled by the government, so Bitcoin is completely Freedom.
Store of Value/Not to be Inflation
Bitcoin works completely against inflation, especially since governments can't regulate it and cannot print it in a printing press like traditional paper money and therefore cannot increase its supply.  So it doesn't get inflation like normal currency. Due to lack of inflation common people can't keep in purchasing power thus Bitcoin's value is maintained.
Bitcoin's Utility & Growing Globally Adoption
Bitcoin's value is maintained as it is exchangeable with various products and day by day various circles, famous people, institutions, countries are recognizing Bitcoin around the world.  Moreover, various business organizations are accepting Bitcoin as a payment system for their business.  Because Bitcoin is such a global revolution, its value will always remain.