Why are some vegetables/fruits seasonal?
Many plants start flowering (and subsequent seed / fruit production) based on light, temperature, and / or humidity changes, which tell them summer is near. That is when they need to grow fruit. Surely, there are plants trying to look for sunlight, pollinators, good conditions for seedlings, in the winter, but they are specially adapted, and thrive in their niche only through some season sensing similar to the summer-flowering plants.
At the Equator there isn't much difference between seasons and days, and there  should be no need for such mechanism. Bananas are suited to the Equator, meaning banana trees don't know what seasons are.
Plants in temperate climates care about seasons a lot. We put them in greenhouses and we play with their light, in order to force off-season flowering. Tomatoes are kept in the greenhouse not because we don't like them shivering outside, but because we want to fool their temperature-sensing organs that summer is here.
At the Tropics there is an important difference in the amount of rain water and a smaller change in day duration. Mangoes are more suited to the Tropics, meaning they care about seasons. People who grow mangoes in greenhouses try to fool them by adjusting humidity, but I couldn't find any evidence that it is humidity rather than day length.