(transformer) why does iron loss in core doesn't affect voltage but copper loss in winding affect voltage?

Electric current doesn't pass through the iron core. The iron core serves the purpose of concentrating the magnetic flux and hence increasing the magnetic coupling between the primary and secondary coils. The eddy current so produced in the core is by electromagnetic induction. Hence, the (eddy) current only consumes part of the power in the transformer, and this lowers its efficiency. The voltage induced in the secondary coil depends only on the "turn-ratio" (i.e. the number of turns in the secondary to that in the primary coils). The same secondary voltage will be produced even without the presence of an iron core. But, of course, the efficiency of the transformer will be greatly reduced.

Resistance in the winding wires consumes electric power (this is the heating effect of current). A copper coil in reality can be regarded as an "ideal coil" (one without any resistance) in series with a resistor. Since an electric current through any resistance causes a potential drop, such potential drop would decrease the voltage across the coil.