Oxidation States of D Block Elements
Actually an organometallic scandium(I) compound is known, from P.L. Arnold at al. (1996).  As with organometallic compounds generally, the metal is covalently bonded, and the use of $3d$ orbitals as valence orbitals (there are 14 shared valence electrons associated with each scandium atom) in this and similar compounds favors including scandium as a transition metal.

In predominantly ionic settings, however, it is true that scandium(I) would be unlikely.  Because scandium is only at the beginning of the transition metals it has relatively low ionization energies for its valence electrons, and this low energy cost tends to be overcome by increasing lattice or solvation energy associated with higher ion charges.  With ionically bonded scandium the balance is all in favor of more ionization and more electrostatic attraction, whereas with titanium the slightly higher ionization energies and the fact that a noble gas configuration requires four rather than three positive charges makes lower charge ions more accessible.