While the reason is — and will probably remain — unknown, it has been officially confirmed that the Android Pie update for the OnePlus 5 and OnePlus 3 handsets will not arrive as soon as expected. Although an official launch date has not been revealed, the aforementioned smartphones will probably receive the update by the end of the year.
https://www.notebookcheck.net/OnePlus-delays-the-Android-Pie-update-for-the-3-and-5-series.338774.0.html