Posts: 27,061
   
Threads: 6,784
    
Likes Received: 4,512 in 4,106 posts
Likes Given: 1,941
Posts: 14,679
   
Threads: 10,281
    
Likes Received: 2,325 in 2,122 posts
Likes Given: 396
The benefits may not outweigh the costs as North Korea is still under US sanctions since the Korean War came to a stalemate in the 1950s (Vietnam was very lucky to not be in North Korea's situation from the 1990s onwards as the US needed Vietnam's help to search for MIA US servicemen in Vietnam).
One way is to link via Sakhalin (only 40 km from Hokkaido so it's a shorter distance than going to South Korea) and then Russian Manchuria (another 6.5km only between Sakhalin and Russian Manchuria) but that whole area is very underpopulated.
Russia did not develop that part much and it is such a big roundabout way. Maybe if Russian Manchuria remained as Outer Manchuria of China, the place could have been developed well but it did not so really no point linking to there.