Because a procedural justification doesn't set the right kind of precedent.

Right, so its a weaker outcome. But if we assume that justices have ideological concerns with policy outcome, there is no reason that the selection to make a procedural decision would be any less a subject to those concerns than the selection to make a merits decision. In fact, the fact that it is a weaker outcome (on the subject addressed by the substance of the case) than a merits decision with similar effect in the immediate case would in some cases be the motivation for pursuing it for an outcome-oriented justice.