if there was no central tapping, what would be the change?
Consider this circuit diagram, from the answer you're not satisfied with:
There is no "central tapping" so the two diodes are connected in series. In a series connected circuit, the current through each circuit element is identical.
Now, note that D1 allows only a clockwise current while D2 allows only a counter-clockwise circuit. The only solution that satisfies both constraints is zero current.
So, without the center-tap, there is zero current in the secondary circuit and thus, by Ohm's Law, zero voltage across the resistor R.