I think you mean most secure, not most stable. It has the first mover advantage, it's well known, it's trustworthy. It's the coin with the largest market capital Technology wise it isn't that impressive, but it's simple and seems to work good enough. Another thing to remember is that a bitcoin's value is not necessarily tied to the effectiveness of the Bitcoin network. Bitcoins can be migrated and live on newer and better projects.

The "moat" is public adoption, because public adoption is going to be the strongest driver of eventual price stability. No other cryptocurrency has as much public media momentum and institutional buy-in.

Momentum being, Square and Paypal bought into Bitcoin, so other institutions bought into Bitcoin, and so Tesla is going to buy Bitcoin, and because Tesla bought Bitcoin, others are going to buy bitcoin. Why? Because each successive buy-in indicates more trust in the cryptocurrency, backed by the financial inertia of large companies.

Of course, down the line, if all cryptocurrencies become more stable, then there could be changes to other currencies with more attractive features. But by that point, it would have gained so much inertia and momentum that it will likely continue being the de-facto cryptocurrency, with other cryptocurrencies having their value underpinned to BTC similar to the way we currently value BTC in terms of USD.