Isn't it an oversimplification to reduce the influence to the rainy season when booking tickets?
We must categorize factors properly, because the influence of weather is conditional, not directly causal.
There is the category of unavoidable constraints (professional appointments, school holidays) where weather is secondary.
Then, there is the category of cost optimization, where price dominates, regardless of sunshine.
A business traveler going to Singapore will care more about their meetings and ticket cost than humidity.