Complex sentence: In the end, Ryzhkov failed in his promise to rebuild Spitak, partly due to the Soviet Union's mounting economic problems, and partly because many of the city's Soviet era buildings had not been designed with adequate earthquake protection, making their reconstruction more difficult.
Split into simpler sentences: In the end, Ryzhkov failed in his promise to rebuild Spitak, partly due to the Soviet Union's mounting economic problems. Furthermore, many of the city's Soviet era buildings had not been designed with adequate earthquake protection making their reconstruction more difficult.