More likely the small size, flight and the holometabolous lifestyle.
There is the theory that the number of species is related to the number of available niches. For mammals, a tree may offer 2-3 with the ground, the branches and maybe something like burrowing (this is just for illustration purposes).
Insects can live in the leaves, dead branches, inside the wood, in the mosses, on the ground, in the leaf litter layer, burrowing etc., etc. because they are so small. They can also easily transit between different places because most of them can fly.
Because the larvae of holometabolous insects can occupy a completely different niche than the adults, every combination of niches can more or less be considered a new niche.
All of this is reflected in the species richness of insects. The primary wingless groups of insects are not very diverse compared to winged insects. And within the winged insects, the holometabolous species make up the vast majority. Hymenoptera, flies and beetles make up the majority of insects and they are all winged and holometabolous. If you just look at the hemimetabolous ones, they aren’t much more diverse than other groups of arthropods.
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