The U.S. Federal Energy Regulatory Commission issued the ruling on Friday.
A key sticking point is Enbridge's bid to set the rate structure that it could charge to transport oil through the pipeline.
The commission noted that it has not supported Enbridge's proposed rate.
It also said Enbridge has yet to present an alternative that the commission can approve as fair, reasonable and in the public interest.
Even so, two of the five commission members dissented from the ruling, saying the denial will unduly delay infrastructure investments in the region.
A call to Enbridge, which is based in Calgary, Alberta, was not immediately returned.