why are these Russian speaking areas part of the Ukraine in the first place? Is there some "USSR related" reason......?
That was a decision of the Soviet leaders, mainly Stalin and Khrushchev.
Is there an "ethnic" difference between Ukrainians and Russians, or is it simply a cultural/linguistic thing...
No, only selfdetermination, which is promoted by the state. Many "Russians" have become "Ukrainians" since the independence.