Because in the adoption phase you have early adopters that acquired coins for such small amounts of money who become very wealthy and can easily live rest of their lives by selling off coins (we are still on a fiat standard after all, so this is the most efficient way to convert bitcoin into goods and services) so when the price rises dramatically you get a lot of additional supply coming to the market from early adopters. S2F to me makes more sense when we get to a larger base of users, there is less volatility and realized price and price trade in a smaller band.