Compare atomic size and ionization energy.

Lithium has a smaller atomic radius and higher ionization energy compared to potassium. This is because lithium's valence electron is closer to the nucleus and experiences a stronger effective nuclear charge.

Reactivity for alkali metals is determined by their tendency to lose their single valence electron. Since lithium requires more energy to lose its electron (due to higher ionization energy), it is less reactive than potassium, which readily loses its electron.
Lithium is less reactive than potassium because its valence electron is held more strongly by the nucleus due to its smaller atomic size and higher ionization energy, making it harder to lose the electron.