The reason is unknown and NVIDIA's official line is that it's cooperating with HTC to investigate the issue. In the meantime, NVIDIA recommends users connect HTC Vive to the HDMI port.
"We are aware of user reports of an incompatibility when plugging HTC Vive into GeForce GTX 1080 using the DisplayPort connector. HTC and NVIDIA are investigating the issue. In the interim, we recommend users connect HTC Vive to the HDMI port on GeForce GTX 1080," said Bryan Del Rizzo, Senior PR Manager, Consumer Products.NVIDIA was first alerted about the issue over a month ago, so far there hasn't been any significant update.