How cool does a place have to be so it’s hot?
The development of this acceptation of hot is quite obvious: You can set the world on fire or burn with desire, so heat is often used metaphorically for passion, which is considered a positive thing.
But cool, in the actual sense quite the opposite, can convey a positive evaluation as well. Here, the development took one more corner: It is a good thing not to show passion, to keep cool, in the face of alarming (or very hot) events unfolding—this is the starting point for a positive reading of cool as serene, which then can be further extended to fashionable and finally agreeable.
Cool and hot thus both can be used positively, but they stand for different traits—compare That guy is cool to That guy is hot. Still it’s remarkable that these two antonyms developed metaphorical uses that are far from contrary.