He wanted the Soviet Union to be easier to rule, and right or wrong he thought making it less ethnically diverse would help with that goal.

That logic might work for recognized ethnicities that didn’t have their own SSR within the USSR after the 1924 constitution changed the right of self-determination from applying to recognized ethnicities to SSRs, but Ukraine had it’s own SSR and the right to secede under all their constitutions. If Stalin wanted to reduce the number of ethnicities, he’d target ones that couldn’t unilaterally secede.