Money. It's about money.
People will pay a huge amount to see the first live mammoth. It captures the imagination in a way elephant or mouse or bird cloning just cannot compare to. I'm all for science advancement and in favor of careful study of biomes and being cautious, but none of those things grab headlines. The money that would pour into de-extinction development for a mammoth will be huge compared to any of the suggested other projects, and that's why cloning a mammoth is such a good thing. Getting the research and testing and trials done to bring back a mammoth will make it tremendously easier to complete other projects that may have more fundamental benefit to the scientific community.
You don't send a mission to Mars or even to the moon because that's the best return on investment, you do it because it makes people care about the science in ways they never would for the fundamental research that makes it possible.
You have to deal with the public like you would a four year old child. You don't have a four year old child look forward to going to the dentist because it's good for their health, you get them to look forward to it because they get a toy and ice cream after. Likewise you don't get the public to support a scientific project to benefit the ecosystems and research that being able to clone extinct and endangered animals can offer; you get the public to support the idea of getting to see a mammoth in person.