Rokan... for years three of the most usual reasons are:
1) Typically, the LV-winding typically carries the heavier current, so it size is much larger than the HV-winding current. And, because the number of turns is low, less is required if the coil is wound closest to the core, that is its cicumferce is smaller.
2) Typically, the HV-winding is tapped, hence easier to accomplish! The tap-off placement accuracy is greater because ther are a greater number of turns are present in a e smaller.
3) Insulation-to-ground is less if the for the lowLV-winding
Today, other arrangements are being developed and evaluated in the interest of lower cost and higher efficiency. One example is the use of 3-coils per-phase: an inner LV-coil, a middle HV-coil, and an outer LV-coil?
