Interesting question. I think @Dot is right. From a survival point of view, it's probably more important that we pay more attention to negative emotions. 'Shit, that looks like a tiger' is more life saving than- the sun feels nice and warm today.
I think as well, when predominantly pessimistic people do experince positive periods, we become afraid of the fall back down. So, it's like- when we're already down, there's maybe a comfort that we can't feel much worse.
It also can take effort to sustain positive emotions or, sustain the activities that produce them. It can feel like a real fight to battle negative feelings. So, I think there's also a sort of lazy comfort in wallowing in melancholy.