Good question... From the report:

Phil Taylor, group director of Open Seas, said: “MPAs are designated to safeguard the most precious parts of our seas. Wester Ross is a rare success story, an MPA where local recovery is visible, with coastal communities already benefiting from the return of marine life after years of decline caused by poor fisheries management.”

He added: “This project risks undoing that progress. It will involve using a water jet to blast through the MPA to dig a trench for a high-voltage cable, directly damaging fragile seabed habitats including those which the MPA was designated for — with impacts expected on some maerl beds, feather star aggregations and potentially even impacting the breeding grounds of critically endangered flapper skate.”

The problem seems to be the 'blasting water jet' part, not the cable itself. It really sucks, due to the suspension, turbidity and deposits it will create in a vast area.