Why are clementines easier to peel than oranges?
I think it all comes down to the thickness of the skin and how it's attached to the fruit itself.  For me, it depends on the variety of oranges and/or clementines, mandarins, etc. that I'm peeling.  Usually fruits with thick, hard skin are easier to peel whereas fruits with softer more delicate skin are harder to peel off in one piece.  Factors like growth climate and ripeness effect the skin quality. 

If you roll the fruit on a hard surface under your palm before peeling, it makes the skin easier to peel off in one piece.