This is a combination of costs and timeline. Oculus Quest development started in 2016 or 2015. Snap dragon 835 released in 2018 and Quest shipped in 2019. Now,

This is a combination of costs and timeline. Oculus Quest development started in 2016 or 2015. Snap dragon 835 released in 2018 and Quest shipped in 2019. Now, Quest shipped 6 months after 865 so it would have been possible for Facebook to co-develop Quest with Qualcomm and potentially ship with an 865, but due to various factors (VR is a small market, Facebook doesn't have a close relationship with Qualcomm like console manufactures have with AMD, Quest needed to be "cheap")