The following factors are increasing the long-term value of Bitcoin:
1) Age of Bitcoin. The longer is the history of some tech, the more people trust it. More people are realizing that Bitcoin is not going to crash.
2) Growing number of Bitcoin users. As there is a limited number of coins, more Bitcoin users means more scarce coins. More users also means that there are more ways to spend your coins, thus Bitcoin is becoming more useful as a system.
3) Bitcoin is becoming more accessible for non-techie people. There are user-friendly wallets, BTMs, good video manuals, Bitcoin debit cards, ETFs etc etc. It's significantly easier to own/use Bitcoin today than 5 years ago. And in the next 5 years it will be even easier.
4) The legal status of Bitcoin is becoming clearer. Some developed countries already incorporated Bitcoin into their legal frameworks (taxation, regulations etc), other countries are working on it. More people can be sure they're not breaking any laws by using Bitcoin.
5) The world is transitioning to the digital economy. Digital money is becoming a norm, thus increasing the trust in cryptocurrencies.
6) Growing number of lost coins. As people sometimes lose their wallets etc, coins are becoming more scarce.
7) Technological improvements in Bitcoin. Bitcoin itself is becoming better: bug fixing, new features added etc.
8) Growing adoption of Blockchain by all kinds of industries. The more people trust Blockchain, the more they trust the currencies based on Blockchain.
9) Growing number of people who distrust the Government and the banking system. The mass surveillance scandals, the banking crisis, the insanity of Venezuela and India etc. Such news are making people seek for alternative solutions.
10) Growing number of Internet users. A half of the world population still doesn't have any Internet access at all. As more people get the access, more of them will learn about the "currency of the Internet" and adopt it.
11) Growing acceptance of Bitcoin as a store of value. The factors #1 and #9 are making Bitcoin more attractive as a store of value, thus increasing its popularity (#2).
What factors did I forget?