Because the US effectively watched your back and saved your country from Indian devastation ( West Pak) and prevented any UN related hassles.
The Blood Telegrams itself were a result of frustration among the embassy officials (Archer Blood) in Dacca over the US Govt's apathy towards the continuing genocide and selected killing of Hindus and Awamis by the West Pak Army and the affliated Al Badr,Al Shams groupies.
Remember you were real homies with the US that time.
Come up with all the excuses you want, no one verified a 3 million figure that the Bengalis claim. India played its part as well in trying to distort the historical facts of the time.