Why do some entomologists classify proturans, springtails, and diplurans separately from insects?

Some entomologists classify proturans, springtails (Collembola), and diplurans as separate classes rather than as members of the class Insecta because they are now considered evolutionary offshoots from the main insectan stem rather than true insects.1

The primary morphological distinction used to separate these groups is the structure of their mouthparts. Proturans, springtails, and diplurans possess entognathous mouthparts, meaning their mouthparts are retracted into the head capsule. In contrast, true insects are characterized by ectognathous mouthparts, which are exposed.23

Historically, these groups were grouped together with other wingless hexapods in the subclass Apterygota within the class Insecta, largely due to their shared lack of wings and presence of six legs. However, modern specialists have shifted away from this classification. By treating these entognathous groups as independent classes, scientists better reflect their distinct evolutionary history and morphological differences from the ectognathous insects, such as the Zygentoma and Archaeognatha.143