Answer
It’s because of the electronic structure of alkali metals.
Work Step by Step
Alakali metals contain only a single electron in the outermost valence shell, so the atom can be considered to be a single electron orbiting a shielded, reduced-charged nucleus. This model adheres quite closely to the assumptions made in the central-field approximation.
The situation in transition metals is rather more complicated and is not described well by the central-field approximation.
