Across the periodic table of elements, we have trends in acidity, basic nature reactivity, metallic, atom size and non-metallic character etc.
Across a period the metallic character decreases as we move from left to right. In a group the metallic character increases when we move from the top to bottom. The electron affinity reduces as we move down the group.
Potassium is the next element after sodium in the sodium group. So radius of potassium is more. The ionization energy is less. So electrons are more loosely bound in potassium. So potassium is more metallic and reactive than sodium.