Answers: I can think of a couple of possible reason ...
1) In the winter, it's colder, so you wear more clothes and keep more of yourself covered up. And aside from melanoma and cancer-related issues, most skin conditions benefit from exposure to fresh nouns and sunshine. So when you stay more covered in winter, something resembling psoriasis tends to catch worse.
2) Depending on where you live, within the winter it's probably less humid than it is for the rest of the year, and dry nouns tends to worsen skin conditions.
Because your skin is abundantly drier. Try some cortisone cream.
Over the counter.