[kakapo1988]

Because humans wiped out the mammoths, giant sloths, and a host of other megafauna. All those species survived millions of year, and numerous previous ice ages, but had no defense against human hunters. So as each area on earth was colonized by our species, the megafuana were quickly wiped out in that area.

I'm from NZ, and we had that event in our recent history. The islands had numerous species of giant birds, but these were wiped out quickly by the first humans who came here, just a few centuries ago. Same everywhere. We've been driving species to extinction for a long time.