Why are the oxidation states of transition metals in their compounds positive?​

This is because unpaired valence electrons are unstable and eager to bond with other chemical species. This means that the oxidation states would be the highest in the very middle of the transition metal periods due to the presence of the highest number of unpaired valence electrons.

Explanation:

Mostly because the electronegativity of these elements is pretty low and to get a negative oxidation number you’d have to let them react with something that is even less electronegative, say potassium or cesium.

Such compounds may well exist but they usually have metallic bonding and that makes it hard to distinguish oxidation states, because these darned electrons keep running around instead of sitting on any atom in particular. Chemists often discard such compounds by calling them ‘alloys’ as if that solves everything.

There is an exception to this story though and that is one of the last transition elements: gold. Its electronegativity is pretty high, about 2.54 which is about the same as selenium. With alkali metals it does form aurides that are not very conductive and gold is in a -1 oxidation state.