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 Kafkas 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.