Rawhide isn't floppy. It provides more than decent protection against most of the weapons the Zulu routinely faced, and is much lighter than wood for a given size of shield. The real problem here is engineering: You can make small and thin wooden and (given good metallurgical skills) even iron shields. Once they get bigger, though, their own weight and rigidity means they become unstable unless they are thicker. A relatively thin iron sheet would provide adequate protection from javelins, but if you make it 1 metre by 1.5 metres, it will buckle under its own inertia before you even hit it. An iron shi8eld that size would need to be so thivck as to be almost unmanageable for structural reasons (Renaissaancve european smiths made 1-metre shields that were iron sheets on a wooden core for that reason). Rawhide, on the other hand, is flexible enough to withstand that kind of stress. The central fold you see on a lot of African shields has an additional stiffening function. All of this at less weight than an equivalent wooden shield - even the light, disposable kind the Vikings used.
Weight, of course, is the other reason the Zuilu went with rawhide. the Romans used shields of roughly the same size (the Greeks called it "fighting with doors"). We have (had?) a surviving one from Egypt, based on which reconstructions have been made. Scuta are massive weapons designed to take punishment and be used in close combat. You may march 35 km a day with one, but you will not run it. And that is of course exactly what Zulu armies did - run. If you are in good shape (as most zulu warriors were), you can do that with a rawhide shield, a stabbing spear and some javelins. Don't try it with a mail shirt, danglium, helmet and 15+lb shield, though.