This is because binding precedent is only set by the appellate courts and while this judgment is persuasive, other judges sitting in this court can ignore it since they're not bound to follow it.
The appellate courts are generally there to resolve legal questions not factual ones—I'd question the Assange case, or other similar cases would raise a similar legal rather than factual question.
There has been a trend towards applying s91 cases for defendants with autism spectrum disorders: the courts didn't in Gary McKinnon's case, but the Home Secretary (Theresa May, before she became PM) justified her denial of the extradition order on that basis. In Lauri Love's case, the magistrates' court allowed the extradition order but it was overturned on appeal in the High Court on the basis of Love's not being able to cope in US custody with Asperger's syndrome. In the Assange case, the magistrates' court followed this established and rather persuasive precedent.
Court originally ruled that extradition can happen, but the appeals court ruled differently, citing that extradition would be "oppressive by reason of his physical and mental condition".
As far as I'm aware the case never made it to the Supreme Court.