Regulation is definitely more effective to reduce volatility, it's need the biggest nation e.g. U.S. to control all of centralized exchanges and force them to open the market for 7 AM to 7 PM and the price difference of the coin can't be higher or lower than 5% in a day, this will make Bitcoin price more stable.

Bitcoin adoption could reduce the volatility if everyone already bought Bitcoin, but the reality there's always a new people want to start to buy Bitcoin due to many factors they can't e.g. lack of money, not ready to buy Bitcoin.

But which person want to see Bitcoin price is more stable? obviously not me.