Bitcoin is used as a speculative instrument because by definition it is deflationary. Which means it is more expensive to buy something with it today, then tomorrow. Just look at Chamath's tweet about how he bought a property for Bitcoin in 2014 for $1.4MM but today that same amount of bitcoin would be $100MM+
https://markets.businessinsider.com/currencies/news/chamath-...
This creates an environment there is never a reason to use the currency for a purchase, strictly because it's supply is completely limited and never increasing over time.
Since the world economy continues to grow as GDP, output, and the population grows, it stands to reason that if Bitcoin in any way parallels our real world economic scenario, it would continue to rise in price per unit, because the overall GDP continues to increase.
As a result, it in fact never stabilizes, unless overall GDP stops growing.
Since it never stops growing, it never makes sense to use it for a purchase, and so you have a flywheel effect of ever increasing upward price pressure.