My guess? the stick part blocks the effectiveness of the explosiveness at where ever the stick is point at time of impact. You could argue that to change that would be to make the stick part the actual explosive part and have effectively an explosive baton. But then the ends of the “danger” baton wouldn’t have the same explosive effect to the target facing the end of the baton grenade compared to a target facing the sides of the baton. So, to mitigate that, a sphere shaped grenade would probably be ideal with separable metalic “scales” as a shell. Now you have ideal coverage of the area no matter how the grenade lands at the target.

At that point, I would assume ergonomics and determining which way was up so that you know where the pin is leads to why grenades look the way they do today.