The sugar is what is feeding the yoghurt bacteria, without it there is no fermentation. All the delicious sour flavours of yoghurt come from the acid which is produced by the bacteria as they work their magic on the sugars.
If you do want lower sugar we recommend leaving your ferment for longer, make sure you properly sanitise before starting to account for a longer time at room temperature. This will give the bacteria more time to consume the sugars present.