The above is purely my opinion and is frequently the subject of disagreement. It is, however, why I got into bitcoin.
If another becomes more popular, everyone shifts to it, and the price crashes, what use was bitcoin as a store of value?
The difference with other assets like cash or gold is that they have value that is supported by a clear rationale based on a central bank’s economic policies.
1. I believe in the technology approach bitcoin is taking over any other coin (I believe functionality like smart contracts should be a side chain to the main bitcoin chain).
2. It has had a firm 21m supply cap since the beginning
3. It is the most decentralized
4. It has the largest userbase -- it's a more widely adopted currency
5. It has the most proof of work behind it (most of the mining power in the world targets the bitcoin blockchain). Proof of work is what keeps the chain secure from malicious actors.
6. The ledger is public
Something like gold can't be audited. If I bought gold, I wouldn't want to store that in my house. I'd need to trust a third party to store it for me. Who's auditing that third party? Is my gold only mine on paper? Bitcoin is auditable, gold is not (for the average person).
Cash is a depreciating asset -- at best, it loses 2% per year (likely much more).
Don't worry, in 18 months^, Lightning Network (TM) will come out and fix everything by adding a second layer to bitcoin, nevermind that the first layer will be ignored
^ Not accounting for perpetual delays as LN as been promised for a long time