Having Nordstream pipelines in place, but turning off the tap, gave Russia a strong bargaining chip for negotiating an end to the sanctions and the war. It was contrary to their interests to destroy it - and that aside from the roughly 800 million Euro value of the gas contained in Nordstream 1, now venting into the Baltic, and the value of Russia's capital investment in these pipelines.
The US on the other hand, or at least the Biden administration, had an interest in ensuring that Germany wouldn't do a deal with Russia. In recent days there have been protests in Germany which might have acquired enough momentum to tip the German Government's position. As well, Biden himself already foreshadowed this.
The US Navy has even been trialling the technology that would be required, in the very area that the explosions took place:
In support of BALTOPS, U.S. Navy Sixth Fleet partnered with U.S. Navy research and warfare centers to bring the latest advancements in Unmanned Underwater Vehicle (UUV) mine hunting technology to the Baltic Sea to demonstrate the vehicle’s effectiveness in operational scenarios.
Experimentation was conducted off the coast of Bornholm, Denmark, with participants from Naval Information Warfare Center (NIWC) Pacific, Naval Undersea Warfare Center (NUWC) Newport, and Mine Warfare Readiness and Effectiveness Measuring (MIREM) -- all under the direction of U.S. Sixth Fleet Task Force 68.
