why the resistances and reactances of the windings are represented as sereis connection whereas the same parameters of core is represented as parallel connection?

Basically the resistance indicates the flow of current and reactance opposition to the magnetic field. Incase of windings, the current responsible for the copper losses is also producing the working inductance. Since the current is same the two parameters are connected in series.

Now in case of core, the current responsible for the core losses and the current required to set up the magnetic field is different. Therefore, the two parametrs are connected in parallel.