The reason? The grandson "lacked standing." Standing to challenge a will, the Court noted, is determined on a case-by-case basis, the general rule being an "interested person" in the estate may challenge, but a "stranger" may not.
How is a grandson not "interested" in an estate that could eventually pass to him (after his parent's death)? The Court decided the real party who would benefit from the challenge was not the grandson, but his mother. Contrariwise, the Court found that the grandson would actually be harmed by a successful challenge. Stretching the common law rule on point slightly, it therefore decided the grandson lacked standing.
Careful reading between the lines in this case reveals that the whole affair probably didn't pass the high court's "smell test." The challenger of the will, the grandson, was actually assisted in the suit by his father. One read of this case would be to assume the mother and father of the challenger were estranged, divorced, or separated, and that the father saw more support to his son in a benefit to the mother. Another might be that the mother didn't want to challenge the will because of its in terrorem clause, disinheriting any contesting beneficiaries.
A safe bet is that the challenging grandson's lawyers were pretty surprised he didn't have standing to contest the will, as he seemed to meet the common law definition of an "interested party" and not that of a "stranger." But the case does serve the laudable role of closing off to some would-be will challengers a way to conduct an end-run around ad terrorem clauses. This is not surprising: too-clever-by-half legal strategies tend to fail, and perhaps they should. My take-away is that I'm glad Georgia wills (particularly in terrorem clauses) can't be defeated by the clever device of "well, then we'll just get our son to sue."