You ever realise you can't use movement on hills to explain a situation in the positive?
You can't tell someone things are going to get better by suggesting it's all downhill, because that's...bad (?) - even though going downhill is such a joyous, carefree, low-exertion process. But, you also can't tell someone it's all uphill, because they'll just think you're scaring them by pointing out the obstacles they have to climb.
It's a conspiracy by the British I tell you. Why else would the most commonly used phrase to explain good times be smooth sailing? What else would you expect from a bunch of islanders whose love for seafaring helped them conquer half the world?