Gold is stable in isolation. If you bury gold in your backyard, and dig it up 20 years later, you will still have the exact same amount of gold. The exchange rate between gold and other things won't change that.

On the other hand, for crypto to keep value, you need people to mine it from to the moment you buy it to the moment you sell it. This has a cost, which is currently paid by newcomers. But if crypto becomes stable (i.e. there are no new entrants willing to pay a premium to get in), current holders have to accept depreciation to pay for mining costs. Since crypto is not backed by any kind of real world asset, there is no other source of profit.

An investment that is only worthwile as long as it attracts newcomers is in essence a Ponzi scheme.