Stick grenades were not only used by Germany.

There are several advantages to the "pineapple" design. If you just check a picture of both types you will notice that the pineapple version is smaller and therefore weights less, is easier to carry and easier to transport. Equipping a unit with several thousand M24 or other stick grenades will cost more for various reason. They cost more to produce and then more to transport, the transport factor will be minor but we are talking about huge armies so it will be noticable.

The stick grenade had some advantages which lost its importance because of the changing face of war. During WW1 being able to throw a grenade far away was a major advantages alot of grenades were just hurrled towards enemy trenches. This factor diminished after World War 1 and today alot of grenades are aimed at windows for example this target was literally non existent in World War 1. Iam no expert on Vietnam and this example is just a guess but i doubt you would take a stick grenade in a thick jungle, the surface is greater and the grenade will more easily land where you dont want it to land, just by hitting a tree or something else, the same is true for urban warfare. Another advantage of the M42 was the difficulty of getting rid of the grenade once it landed in your trench, just by looking at a stick grenade you are faced with a more difficult situation than with a "round" pineapple grenade. How to grab? Even thinking about this will cost time. On the other hand the timer of stick grenades were often longer because they were intended to be thrown over longer ranges.

Still, the main advantages of "pineapple" grenades are cost and the lacking neccessity of long distance throws compared to the time when stick grenades were first introduced.

Ignoring the point that the Soviet Union, Finland, and both Chinas used similar "grenade-on-a-stick" weapons, the "potato masher" does have one main drawback.

They're awkward. That stick makes it more difficult to carry than a pouch full of "pineapple" grenades, or hooking them to your belt/harness/whatever. It would be a rare infantryman indeed who would willingly take fewer grenades into combat.

To validate what /u/When_Ducks_Attack said, I just translated a Finnish army (3rd Division) lessons learned document from 1942, discussing equipment for long range patrols and raids. It noted specifically that LRPs had not even tried "potato masher" grenades because they were "awkward to carry." Preferred grenades were French and Finnish "egg" or "pineapple" grenades; these could be carried in snow suit's pockets. It's not as much about the weight as it is the design. (IIRC blast effect potato mashers were not heavier and could in fact be lighter than fragmentation pineapples due to lack of thick metal fragmentation shell.)

(original post and sources in this thread.)