First let us define what is stable?
firmly fixed or not likely to move or change:For example:
If the foundations of the house aren't stable, collapse is possible.
After several part-time jobs, he's now got a stable job in a bank.
The hospital said she was in a stable condition (= not likely to get worse) following the operation.
ECONOMICS, POLITICS a stable economy, country, system, etc. is able to continue in a regular and successful way without unexpected changes:a stable economy/country/government
financially/economically/politically stable
The company must now focus on operating a more efficient and stable supply chain.
stable jobs/employment A proposed merger has resulted in key staff quitting in order to find more stable jobs.
Meaning from
https://dictionary.cambridge.org/us/dictionary/english/stableStable coin is unmovable, unchangeable, because fixed matters the most.