Why is bitcoin "deflationary" ?

Why do people say that bitcoin is deflationary? I understand that for the foreseeable future bitcoin has exponential growth potential. However, lets say that in some hypothetical down the road the total market cap of bitcoin truly does become a global currency and is representative of all global assets and wealth. Would the price of bitcoin not simply come to a stand-still? I suppose the argument is that the population will continue to increase resulting in an ever increasing demand for a stagnant supply? Or is the idea that people will continue to prioritize saving btc over spending btc, meaning that prices will have to continue to drop in order for people to actually spend it? Or is the idea that it can never truly become totally representative of all global assets meaning that it always has room to grow?

So in a btc economy prices would be continuously dropping? Forever? I just can't wrap my head around that idea.