For having the slogan, "Never settle," the OnePlus 2 has caused owners to settle for less quite often. The phone itself wasn't well-received, due to the lack of NFC connectivity and other problems. And despite initially promising Android 7.0 Nougat for the OP2, there have been no releases or further news, leaving owners annoyed to say the least.
In a statement to Android Authority, OnePlus has confirmed the OP2 will not receive an Android Nougat update. The company stated, "When we built the OnePlus 2 we didn't have the software infrastructure team in place we do now." If you recall, OnePlus and Cyanogen, Inc had a blowout after the release of the OP1, leading the company to quickly develop its own OxygenOS ROM for the OP2.
The combination of the former OxygenOS team with the HydrogenOS team may be the primary reason for the OP2's lack of updates. Either way, there are plenty of custom ROM options available, such as LineageOS.