Because passing merchant vessels attract less attention from ASW assets than submarines? At a wild guess, there'd be multiple merchant vessels travelling past

Because passing merchant vessels attract less attention from ASW assets than submarines? At a wild guess, there'd be multiple merchant vessels travelling past the point where the pipes were attacked. Who would notice if a group of divers happened to leave a ship as it transited? And of course, those vessels could be flagged under many a convenient jurisdiction. Liberia used to have the world's largest merchant marine, theoretically. Whereas if you go in with a submarine, and it gets detected in the vicinity, it's far harder to plausibly deny that your country wasn't involved. TL;DR - you're assuming a submarine would be stealthier, but there's a lot of stealth to be had from hiding in the crowd.