Strange rankings.
There are many better welterweights with no belt:
Thurman, Porter, Kavaliauskas, Mikey Garcia, Jessie Vargas, Avanesyan, Ortiz jr, Boots..
I didn`t mention Jamal James and Ugas, because they have some WBA belts..
I believe Quadratillo was the mandatory challenger from what I recall. Since Spence was injured they ordered a fight for the interim-title between him and Lipinets, which got a long delay due to pandemic, and then Qudratillo fell out due to visa issues
Governing bodies' rankings don't reflect the best fighters in the division. They reflect fighters following predetermined paths towards the specific belts of each body.
TBRB, the Ring, ESPN, and BoxRec are the ones who rank the best overall in each division. The governing bodies only care about sanctioning fees.