Why is Australia's big herbivore so strange?

Across the world all the continents and ecological regions have their own large herbivore like bison, elephants, or the guonacos and tapirs in south America. Despite the many differences between each of the continents largest animals, they are all at least partly hoofed, four legged and move around by walking or galloping. However, in Australia the largest animal on the continent is the kangaroo which is none of these things so why did Australia's largest herbivore evolve so differently to every where else?

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Sources:
https://journals.biologists.com/jeb/a...
https://onlinelibrary.wiley.com/doi/f...
https://journals.plos.org/plosone/art...