Craig Hilton-Taylor of the International Union for Conservation of Nature says enforcement was weak and even poachers who were caught weren't sentenced, meaning there was no deterrent.
The Western Black Rhino, like other rhinoceros species, is sought for its horns that are used as trophies or for traditional medicine.
Hilton-Taylor said Friday that the last confirmed sightings of Western Black Rhino was of 10 animals in Cameroon in 2000.
A survey in 2006 found no trace of them and the subspecies was declared officially extinct by Switzerland-based IUCN this week.
