The reason why is a few things, first, they have to make sure that the organism is related, and that they can see that. They don't wanna assign a bird as an insect on accident, so they do tests and then determine. Second, the more factors the better, if you consider all the species in the world, and what makes them different from eachother you would have a very long list. And that is it.
I hope this helps. If you need clarity or anything just ask.