Simply put, because one is soluble and the other is not.
An alkali is a substance that dissolves in water to give a basic solution, i.e. a solution with a pH greater than 7.
In your case, potassium oxide,
Sodium hydroxide will then dissociate completely in aqueous solution to form potassium cations,
So, dissolving potassium oxide in water will increase the hydroxide concentration, making the solution more basic
On the other hand, copper oxide is considered to be insoluble in water.
This means that the pH of the resulting solution will not change