3 important reasons:  Stability - It’s a shame to knock over a good whiskey. No point crying over spilled milk but, a spilled Jameson will have you close to tears. Durability - Many whiskey cocktails require some type of mashing, muddling or mixing of ingredients, and thick bottoms don’t crack. Heat Protection - Glass is a great thermal insulator. Thick glass will keep your drink cold and allow ice to last longer. Science, you’ve done it again.