@Tracey121 because ITV have added them. It's coincidence.
I thought ads had always been shown at the beginning and periodically during on-demand content on the ITV Hub as that was their revenue stream for providing a free catch-up service? At least I've always seen them. They do have an ad free service called ITV Hub+ but that can't currently be used on NOW TV boxes.
If you've managed to avoid them so far then be thankful, some of us have suffered plenty!