Part of that phenomenon is down to the limited budget for Mode Shift Revenue Support funding from the DfT. 'Medium distance' intermodal flows are just where it comes in handy. Some flows get it, others can't. (This is intended as a general statement, not specifically about whether any Wentloog flows are or aren't eligible.)They must go on the road somewhere in South Wales because there's no Tesco facility at the rail terminal, so that might not be anything to do with the contract loss.
It isn't really surprising to hear the contract has been lost though, rail generally does well on long distance flows, and Daventry to Newport isn't that. Though the Midlands to Southampton/Felixstowe isn't much further, and those trains have managed to hang around for quite a while.