, well, i think it's because of some of reasons that's actually can be understood in right way only in complex, but anyway, let's try divide them.
The first reason is: Kim-3 has no of atomic bomb. For sure. It's obvious here for everyone, even for school students. I really don't know what exactly demanded so many of Americans to believe in those "nuke tests", but it causes only laugh Russia. Please, find the details about those tests. Bad Korea says they provides subterranean explosions in 10 km deep (for safety of people, of ecology, of industry and bla-bla-bla) . Do you believe in this? Do you? Alright, now look at this: http://en.wikipedia.org/wiki/Kola_Superdeep_Borehole
The Soviet Union, one of the World's most powerful economies there spent more than 20 years to drill it. How could Kim-3, who can't even feed own people well, to drill 10 km deep hole only for atomic bomb test? Doesn't it sounds a bit... hm... absurd? Do you ever understand what kind of equipment must be designed and manufactured to get in 10 km deep under ground level? I think only drilling process from engineering point of view will be much more complicated problem than making of atomic bomb itself.
And then - where's Kim's industry that would let him to create enough of active material to exceed critical mass? I think you understand at least in basic level how much is needed to be done and created to get combat enriched uranium. If no - try to investigate this question and you will definitely understand that it is absolutely unreachable for Bad Korea, that even can't manufacture their own city cars. Yes, Kim has couple of old Soviet-built nuclear power plants, but there's technologies are absolutely not appropriate for making of weapon isotopes uranium or plutonium even in minimal quantities. More... where from can he get a raw materials? China and Russia would never sell Bad Korea anything like that. Maybe US did it? Yeah, sure... The worst think that Kim-3 can do is to create some sort of "dirty bomb" with common explosive and lot of radioactive waste. But he won't do this. He is crazy guy, but not as crazy to do this.
That's why i declare: Bad Korea has no of nuke. And will never have. All their "nuke program" is just a farce from very begin to very end, they just popping air balloons and singing odes in own honor. But somebody believes and makes own pants wet. Somebody doesn't believes but sell weapons to that ones with wet pants who believes. Nothing personal! Just a business!
The second reason is: i think it's not just up to Russia. I think positions of Russia and China here is the same. If Good Korea with support of the US should unite to Bad Korea by any way (peaceful or military), then Good Korea will be leader, imminently spreading American influence into deep of Asian continent, at least in territory of Bad Korea. It should create very dangerous Geo-strategical situation when the United States would get overland access to Chinese and Russian borders in Continental Asia. It is not just undesirable for continental Asian countries, it is also directly against interests of both Russia and China who thinks that there must be no of American military influence in Continental Asia. Even presence of US advisors in Good Korea makes Russian and Chinese pretty nervous. Any discuss about re-union of Korea would be possible only when US troops completely leave Good Korea. Discuss can be, but without America, please. By opinion of Russia and China, the United States has no right of voice in this region of the Earth. And don't forget about another "complicated regions". Taiwan for China. Ukraine for Russia. Don't forget about problem of Kuril Islands (where the USA directly took Japanese side).
That's why Bad Korea is now needed for both Russia and China as some sort of "partition" between those guys that were called as "Yankee bootlickers" during the Cold War and "good guys" (well, of cause those "bootlickers" thought that exactly they are "good guys", lol). I sure that Peking and Moscow very carefully monitoring the situation in Bad Korea, treating Kim-3 and his bunch of "Christmas tree generals" as a little crazy, but in fact harmless communist fanatics that playing very important role, don't letting the United States gain a foothold in Continental Asia. And Peking and Moscow would NEVER let Kim-3 even to start his nuke program seriously.
The United States leaders themselves need to grow those myth and hysteria about Kim's nukes! Alright, if there won't be Bad Korea, then what? Far East region will suddenly became much more calm and peaceful! No threats, no problems. Then who should buy American SAMs, tanks, combat planes, and helicopters... well, even guns? How Pentagon would then explain their presence in the region, spending gigadollars of tax payers? Let's took a simple example. Did you ever see American M16 assault rifle? Those twopenny-halfpenny junk's manufacturing cost is $50 at maximum. Do you know how much it cost to an army that purchases it? Sure, you don't. Just visit closest gun shop and look. Same thing happening there to more complicated weapon systems, that must be not just manufactured, but also sold for profit. But there will be no profit for that guys who manufactures weapon if there will be no threat. Nobody wants a war, but some of interesting guys needs hysteria.
So, dear friend, just drink something hot and sweet and forget about those panic around Bad Korea as scary dream. All of it is just a stupid farce, scary folk tale for feeble minded hamsters, who spend all their evenings in front of TVs, unable to analyze and systematize even basic information. But we both belong to Homo Sapiens species, right? We have enough of gray matter inside of sculls to understand where is the trick and say "It's a trap!".