There are many reasons and some of them are discussed below
The number of turns on the HV side is more therefore control range of voltage through tapings are more i.e fine voltage regulation can be obtained which on LV side is not possible
For high rating transformer the value of current in LV winding is much higher as compared to HV side so practically interruption of this high value of current in LV side is very difficult as compared to HV side.
Also the LV winding is kept inner side (nearer to the core ), where HV is kept outside the LV therefore tapings can be easily taken from HV instead of LV winding.
For step-down transformer the voltage on LV side increase during light loads, In order to decrease this voltage taping are arranged such that numbers of turn in tapings are large. This large turns decreases the flux which decreases the flux density this reduces the core loss which in turn increases the efficiency of the transformer.
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