By the prices of stuff going up in traditional money, it encourages keeping the goods and trading them, for example if someone owns a house that house's value won't just go down with time, it can go through time. But if goods were to keep getting lower in prices then it can as a result discourage buying them and trading. Which makes Bitcoin unusable as money. To keep your money safe of course you can store in a bank as they give you a small amount of interest.
Bitcoin has a limited supply and as time goes by and its popularity increases the prices can go higher and higher, making it less effective to be used as usual money. Imagine how would a shop keeper feel as their good will only decrease in price. And like traditional money which has banks Bitcoin doesn't offer much solution to this.
So even if bitcoin prices were to stabilize, wouldn't it be more like gold than a currency to use like traditional money? Gold is very similar in those terms to Bitcoin.