The reason is logistics (as usually the reason for most things military related). You can fit a hell of a lot more conventionally shaped grenades in a crate than you can with stick grenades. Even on a person, conventional grenades weigh less, are less cumbersome, and you can carry more compared to stick grenades. Stick grenades really only have their ease of throwing over a conventional grenade

can give you pleasure

ofc anon thought of that

If I remember correctly, the spherical design was mainly created to resemble a baseball. Since most US citizens knew how to throw a baseball at the time it would mean they didn’t have to train soldiers as much