India used the Iran card with US because India was highly reluctant to join the project in the first place.
The prime reason for India backing out of this project is that it would be running overland in Pakistan.
1. Pakistan cannot truly assure the safety of the pipeline running across the width of Pakistan regardless of what it signs on the paper. They throw up their hands and point at 'non state actors' at every moment. Any agreement with Pakistan is not worth the paper it is signed on.
2.Secondly, Pakistan is not a stable country. There is no guarentee there would not be a coup, or some leader or Mullah gets a fit and decides to shut down the supply to India.
3. Even Iran recognized this problem, which is why it denied India's condition that India would only pay for the quantity of gas it actually recieves on its border, not the gas that Iran releases from its end - and Iran would stand guarentee for India recieving as much gas as it has contracted with Iran.