- Availability of genetic material (for extinct animals) can be widely variable. For something like mammoths, we've been able to extract genetic material from frozen arctic samples; one would have to assume that samples as large as mammoths are more likely to draw notice and researchers than, say, a preserved mosquito (Jurassic park notwithstanding).
- Proximity to existing species' reproductive biology: we know _a lot_ about how to manipulate reproductive processes in large placental mammals, and most of this sort of cloning is performed mechanically by harnessing the reproductive process of a host with similar biology (in this case presumably by implanting an embryo in a modern elephant for gestation).
- Availability of genetic material (for extinct animals) can be widely variable. For something like mammoths, we've been able to extract genetic material from frozen arctic samples; one would have to assume that samples as large as mammoths are more likely to draw notice and researchers than, say, a preserved mosquito (Jurassic park notwithstanding).
- Proximity to existing species' reproductive biology: we know _a lot_ about how to manipulate reproductive processes in large placental mammals, and most of this sort of cloning is performed mechanically by harnessing the reproductive process of a host with similar biology (in this case presumably by implanting an embryo in a modern elephant for gestation).