Are you sure that's what they're asking? Pretty sure they mean how do we have more wooly mammoth DNA than dodo DNA as in, how do we know more of their genetic code?

In which case it's as issue of preservation, since wooly mammoth remains are often frozen in the tundra, whereas dodo birds lived in a tropical climate which does not lead to good preservation of organic remains.

I think the OP means just the DNA samples of the species alone, and not contained within a human genome.

It’s not just when the two species died, it’s also a factor of where/how they died. Mammoths are from cold arctic regions, and many of them died and have been encased in permafrost, which is essentially a natural freezer which helps preserve any samples.

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