Several reasons.

1. The philosophical. This is something that is unique and took billions of years to evolve to its present state. There’s nothing else like it in the universe.

2. The practical. It plays a unique part in the ecological web of life. In many cases organisms are reliant on other organism for dispersal, reproduction etc. The removal of a species is like pulling a Jenga block from the ecosystem.

Earth’s eco system is awesome. The chances of any species existing in its current for is minuscule. We should mourn their passing.