Um, the ark wasn't 'needed' for the old covenant, as it was established before the ark was constructed, and the comparison escapes me, sorry. The commandments weren't the covenant, they were symbolic of it, the most direct and lasting gift from God to his chosen nation, held with other sacred relics of His direct aid, in the ark, on an altar. They were outward signs of a divine reality ("sacrament"). I feel that the new gift of Himself eclipses the old, and so it made sense to me that the passing of old to new would be accomplished in the sacrament of the outpouring of His Blood - sign of the new and living covenant - onto the ancient altar that requested the old and held the venerable gifts. Nothing else, just sacramental closure.