why does the same restriction enzyme have to be used to cut the donor DNA out and open the plasmid?

We want the donor DNA to be incorporated into the plasmid.  Using restriction enzymes is a way of creating overhang ends (sticky ends) that have complementary sequences so that the two molecules will base pair.  Each restriction enzyme produces a different sequence in the overhang.  If one uses different enzymes for donor and plasmid DNA, the overhangs will not be complementary and the dnor will not base pair with the open plasmid DNA.  Only when the same restriction enzyme is used on donor as well as plasmid, the overhangs of the donor and plasmid will match up complementarily and all that is left to is DNA ligation to have the donor integrated into the plasmid.