Phthalimide is acidic. When it loses a proton, the negative charge can delocalize to the carbonyl groups. So, phthalimide potassium salt is easier to prepare than benzamide potassium salt.
Thanks for posing another question, but I don't see the link with the reactivity of hydrazine with the carbonyls (in either phthalimide or benzamide, being present in the same molecule)?