Why is scandium not considered a true transition metal?

This is to do with the definition of a transition metal (TM). A TM is a metal which loses electrons to form an ion with an incomplete D subshell. Scandiums most stable ion is a Sc3+ ion losing both the electrons in its 4s shell and one electron in it's 3d shell. This points out that the 4s shell loses the 2e- before the 3d, due to the orbitals being higher in energy, then the 3d e- is lost to form the Sc3+. With no d-electrons the definition doesnt apply to Sc hence not a transition metal. The same is said for Zinc, which forms a 2+ ion as its most stable ion and has a complete d-subshell.