The USSR didn't "decide" to do so, it was couped and hollowed out by Yeltsin and co. These are the new "oligarchs." While the economy had started to slow, the combination of the devastation of World War II resulted in the deaths of 27 million soviets, many of which were some of the most dedicated to socialism and defending it. This was also combined with growing nationalist movements, often supported by the west. This complicated mish mash gave favorable conditions for a coup, despite popular support for retaining the soviet union.