Because it has no historical existence whatsoever. 'Ukraine' means borderland, and that is what it is. Ukrainians have historically been called Little Russians. The current state borders were drawn by Bolsheviks, and include Polish lands and Russian lands - the infamous nationalities policies that was used to divide and conquer, which can also be admired in Karabach.

Of course, you can dispute what makes something a non-country, but this is sufficient to me.