Two reasons: Safety is our top priority. Blendtec blenders are among the most powerful on the planet, and their jars have to handle high-speed impacts from very hard ingredients like frozen fruit. Glass jars have the potential to shatter not only when dropped, but also during high-speed blending. Blendtec's Tritan copolyester jars are engineered to withstand that kind of stress. Lighter is better. Blendtec jars are very light and are molded for easy gripping and storage. Glass jars, on the other hand, are heavier and more difficult to grip, pour, and store.