Priority is given to glass that can be ground and has the correct optical properties. Toughness might just happen with those things, but its not first priority. Getting glass that is hard and grinds well also probably makes it a bit brittle. There is sure to be some tradeoffs. That front element usually does not have any tough refraction demands, so its likely selected for toughness. With some lenses, it is merely a protective element to protect the fluorite which is very fragile.