Another reason for poor fragmentation of the Mk. 2 grenade, was that one of the fillers was EC blank powder. Granted, it was a fast burning powder, but it doesn't give the detonation wave approx. 25,000 ft. per second that TNT gives.
Manufacturing fragmentation bodies is an economic and scientific problem. Mk. 2 grenades were made by the millions, cast from iron or steel because the U.S. got rushed into the war. Money is always a consideration when selecting materials for warheads. Payback in the value of the item destroyed versus the cost of the warhead. We might pay a lot more for a specific Hi-Frag alloy for antiaircraft projectiles to destroy costly airplanes, but scrimp on price when it comes to a grenade to kill a single soldier, especially when we need to make millions of grenades versus 50,000 hi-frag projectiles. As technology and metalurgy improved, ordnance followed with more appropriate materials. The bodies of U.S. 81mm mortar shells were made from pearlitic malleable iron a few years ago, because it fragmented well. The current trend (APAM) is to go to a design using tungsten BBs suspended in a plastic matrix for mortar bodies. Initial testing was very impressive where a 60mm tungsten matrix mortar projo had the same or better affect as the older design 81mm iron projo. The problem with this round, is the high cost of the tungsten BBs.