The carapace spear, which is currently the best pure physical damage one-handed weapon in the game, would probably be quite broken if it had any longer reach. It's very easy to stagger virtually any enemy with it thanks to high attack speed and great amount of damage.

Reverse grip provides more force and also makes it faster to throw the spear. It's also much more convenient for animation of the two attacks.

The grip and reach could be different, but then the weapon would delve into realm of two-handed weaponry in the game's context and require more complex animations and other rebalancing.