Because it's a 19-Nor, didn't realize the joint soothing affects would be completely negated by the short half life.
Just because of it's a 19-nor compound doesn't necessairly make it "good" for your joints.
Is tren well known for that purpose for instance? No, if something it has shown some decrease in collagen synthesis so just assume that all 19-nor's will have nandrolones effect on the joints will not be too wise.
It's possible that Trestolone have some lubricating effect on the joint's, I don't know. Possible.
But if you look at those compounds who has shown most positive effects on collagen synthesis like, EQ, Deca, Primo, Anavar they share the similiarty that they are high anabolic and quite low androgenic.
If this is just a coincidence, I don't know, but Trestolone is a strong androgen so I wouldn't count on any special 'healing' effects from it, just because it's a 19-nor compound. Just my 2.