Why did Gregor Samsa turn into an insect of all things ?
The way you have formed your question is a large part of the answer: we automatically ascribe a “category” or “taxonomy” to his condition, placing him in a physiological family, called “insecta,” simply because he appears to have an exoskeleton. This is Kafka’s point: we humans think in terms of some organized, conscious system designed by a Maker. But the novel is really a fictive statement of the principles of existentialism (“existence precedes essence”) – we design ourselves by our choices, by our conscious decisions and their consequences. Had Kafka chosen another family – mammalia, for instance – his point would not be made as strongly, because we, as mammals, would not perceive the “metamorphosis” as so symbolic.