nothing, it's just that buprenorphine has certain advantages that it lacks: because it's a partial agonist, it causes less respiratory depression/is much harder to overdose on/you can safely titrate it faster; high receptor affinity means it largely blocks the action of other opioids (to the extent that it can be used to reverse overdose https://ccforum.biomedcentral.com/articles/10.1186/s13054-02...), longer half-life at high doses so you can maintain smoother levels/administer it less frequently. this is more speculative but it also seems to have an antidepressant effect, & in general I think it's less harmful to cognition & mood than other opioids incl. methadone. doesn't work better than methadone for everyone but availability issues mean we're still not at the point where that decision can always be made on a purely clinical basis