Full Idea
Originally you count substances for ontology. Once there is the doctrine of real accidents (in the 14th cent) the list of ten categories begins to look like an inventory of the kinds of things there are, and 'Categories' looks like the fundamental text.
Gist of Idea
Once accidents were seen as real, 'Categories' became the major text for ontology
Source
Robert Pasnau (Metaphysical Themes 1274-1671 [2011], 12.1)
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.222
A Reaction
Prior to this development, 'Categories' was treated as a mere beginner's text, once the major corpus of Aristotle had been rediscovered in the 13th century. The result of this development is sortal essentialism, which I don't like.