Why not just use a land cable that goes around the Sea of Okhotsk? A land cable is safer, and tapping it is harder. I screen cap Google Maps. I know the undersea cable is more direct, but did it speed up communication that much? Operation Ivy Bells - Wikipedia.
In the early 1970s the U.S. government learned of the existence of an undersea communications cable in the Sea of Okhotsk, which connected the major Soviet Pacific Fleet naval base at Petropavlovsk on the Kamchatka Peninsula to the Soviet Pacific Fleet's mainland headquarters at Vladivostok.:172 At the time, the Sea of Okhotsk was claimed by the Soviet Union as territorial waters, and was strictly off limits to foreign vessels, and the Soviet Navy had installed a network of sound detection devices along the seabed to detect intruders. The area also saw numerous surface and subsurface naval exercises.
Laying a land cable would be prohibitively expensive. The main reason is not the large distance but the fact that most of this territory is completely undeveloped: it has no roads. If you look at the Google map carefully you see that there is still no roads there. This is a thinly populated wilderness.