At the soil part, do you mean like worms? Because usually animals don’t have much business with the soil
Whell, adaptability is usually reduced to finding food. Mamals are a lot bigger than insects so they require more food. In tough regions where food is scarce insects can live and mamals can’t
or do you mean in deserts and such- where it would be the lack of water mainly, and lack of food (thats why you get so many more insects in the wild because they dont need that much)- But mammals also face the problems of heat and humidity.(i.e storing water)