Why cannot a bhikkhu receive food offered by bhikkhuni?

In Patidesaniya, one section in the Patimokkha, we find such a prohibition. Checking in the Vibhanga, where we learn the historical context of the rule, we found an interesting story. An elder bhikkhuni of 120 years old went for aims in the city at the distance of 4 to 5 kms. Upon her return a young monk was waiting with his empty bowl. Out of respect for monks as prescribed in the Garudharma, she reverently offered him her alms received for that day. The young monk got an idea of not having to go all the way for alms himself and received alms from the same nun on the following day also. On the third day, the bhikkhuni went for alms in the city. While roaming in the city a chariot passed near her path. She took a step aside, fell down and fainted. The millionaire who was riding that chariot came out to make inquiry and learned from her that she fainted out of hunger and tiredness, as she had not eaten for three days. Upon learning the reason the millionaire criticised the young monk and later brought this to the attention of the Buddha. From then on, to protect the nuns from being taken advantage of, the Buddha laid down the rule for the monks not to receive alms from bhikkhunis.