When, the diode is forward biased during the positive half cycle, the resistance of the diode will be low, and therefore the current will flow through the branch of the diode without any obstruction i.e. the branch will be short-circuited and therefore the voltage at output will be zero.

However, during the negative half cycle the diode will be reverse biased and its resistance will be infinite and it will not conduct and in this case the voltage at the output will be the negative cycles.