Frankly, it's because of the political circumstances of their benefactors. In East Germany (the DDR)'s case, they lost the support of the USSR for the division. In 1953, East Germany revolted and Soviet tanks rolled through and crushed it--it was clear from fairly early on in the life of the DDR that it was not a legitimate government but was in fact in place due to the support (by violence if necessary) of the USSR.

In the 1980s, that changed. The USSR was the primary muscle for, and controlling force behind, the Communist Parties, foreign policy, and domestic policy for the Eastern Bloc. If any change was attempted, the USSR would block it, and repeatedly demonstrated (in Hungary and Czechoslovakia after Germany) that they would be happy to use force. Once their threat of force was gone, the end came rapidly.

That threat ended when Gorbachev declined to use force after the June 4, 1989 Polish elections led to the clear loss of supremacy of the Communist party. The Brezhnev doctrine, that the USSR would use force to maintain the supremacy of Communism in the Eastern Bloc, which was first articulated (though not first acted upon) after the Prague Spring in 1968, kept many protesters off the streets.

Once that threat was gone, there was simply no way to support the governments that had depended on it for decades, and the entire system collapsed within the next two years. The DDR needed outside support to remain upright, and without it, collapsed.

Now, in 1989 in China, the Tienanmen Square protests, rather than being allowed to peacefully proceed, were brutally repressed, and the Chinese Communist (we'll ignore all debate about using that word to describe China today) government was able to remain in power. China was (and is) to North Korea (the DPRK) what the USSR was to the DDR, and was (and is) willing to use force to maintain its sphere of influence on the Korean peninsula, just as the USSR was willing (until 1989) to use force to maintain its sphere of influence in Eastern Europe.