UNODC list of fentanyl analogues

[Fertile]

https://www.incb.org/documents/Global_Projects_OPIOIDS/INCB.GRIDS.OPIOIDS.Fentanyl-Rel_Subs_list.pdf

What I find interesting is that the above list is still only a fraction of the fentanyl analogues found in patents. I just cannot fathom why the fentanyl class is still adhered to. It's not like their aren't scaffolds with similar activity.