That's the general question on all the European dress code that guys wear today. Whether it's white shirts/black pants, hats, coats, boots, etc., you can be guaranteed that gedolim of hundreds of years ago never wore such things. The point is to dress respectfully and have "dress cloths" as defined by the time. Even in 2015 there are dress clothes, including suits and tuxs, but sticking with fur hats, especially in hot climates/in the summer seems silly to me.
Even in Europe in the 40s/50s when the fedora came in style, people only wore fur ones in the winter, and many wore straw ones in the summer if they could afford it. No where that I'm aware of does it say frum men need to be sweltering in the summer wearing European winter clothes.