The issue is that the decision was not final; Chief Justice Roberts was still trying to persuade other justices to adopt a split-the-difference decision (uphold the 15-week law but don't kill Roe v Wade). Justices have switched sides before. Leaking it could kill any quiet negotiations. There was a second leak after the first one about this (about Roberts' position).
So someone on the hard-line anti-abortion side had a motivation to do the leak.
So someone on the hard-line anti-abortion side had a motivation to do the leak.