Because over time total productivity increases and the nominal cost of production falls. So you're always in the position of representing the value of *more stuff with the same quantity of money.
You end up disincentivizing economic activity because it's better to hold your money unless you have some surefire get rich scheme.

In other words, a little supply inflation is good acchually. 

 I am of the view that a lot of products are just bought because fiat doesn't gain or at least not loss purchasing power. I don't like the throwing away society. With a disinflationary currency, you will have less incentives to do so.  

 People will spend no matter how disinflationary the currency is because they need some food and other things. Nobody will do hunger strikes because the money is gaining purchasing power for example. People will be concious when they want to buy something.