If the price of Bitcoin can easily go 10x or lose 90% in any given year then it becomes impossible to use it as a measurement for value. Imagine if you had a ruler that kept changing the definition of what a foot or meter was. That ruler would be worthless. You'd never be able to build a product using those measurements.

The same is true for an unstable currency: it can't be used as a measurement and therefore becomes a worthless unit of account. This is why most of the world values USD so much: it is the most stable currency available to them by exponential margins.