I’d say it was a combination of aesthetics and functionality.
Aesthetically, the iPad mini doesn’t NEED a cover, because its small and light. A cover would protect the screen, but… It would “thicken” the device and (in my opinion, and potentially in the opinion of Apple!s product managers) make it look less sleek and add bulk.
Functionally, the full iPad Smart Cover is also a very useful stand… And the iPad mini is so small you don’t need a stand.
Not to say that they won’t eventually ship a “mini Smart Cover”… If Apple sees a market opportunity (I.e. if other companies start successfully selling their own mini covers) they will very possibly release a product to take advantage of it.