This is because Best Effort traffic is all other kinds of non-detrimental traffic that are not sensitive to Quality of Service metrics (jitter, packet loss, latency). A typical example would be peer-to-peer and email applications2. Background traffic is a type of traffic that is used for system maintenance or backup purposes and does not affect the performance or availability of the network3.
Therefore, Best Effort traffic has a higher priority than Background traffic in terms of network resources allocation and management.
1: https://www.arubanetworks.com/techdocs/ArubaDocPortal/content/docportal.htm
2: https://stackoverflow.com/questions/33854306/best-effort-traffic-and-real-time-traffic-difference
3: https://www.informit.com/articles/article.aspx?p=25315&seqNum=4