She announced this herself on social media
"After Garant’s extraordinary effort in the first round of today’s Nations Cup in Rotterdam, Beezie, John, and Mrs. Wexner have decided to withdraw Garant from consideration for the US Olympic Team this year. We all believe strongly in Garant’s future as a championship horse, but know it’s not the right time for him yet."
Further she also states that she hated to let the country and the team down, but "the only think more imporant than the team is the horse.