There were several reasons for this, the Soviet Union went from initially tepidly in favor of intervention to firmly against intervening.
Unlike in 1968, the Soviet Union at the time was already engaged in a war in Afghanistan which was escalating. Not only did this serve as a distraction militarily, but also politically it warned the Soviet politburo that an intervention might not be as quick and smooth as 1968. And in fact if the Polish choose to resist and the elements of the Polish army rebel it would lead to a bloody, expensive conflict.
The Soviet economy was already strained both by the war in Afghanistan itself as well as western sanctions in response to the war. The United States warned the Soviets that sanctions would be toughened should the Soviets intervene.
The existence of a government in Poland which could theoretically suppress the anti-government movement. Unlike in 1968 when the Czech government itself strayed from the Soviet guided version of Communism under Dubcek, the Polish government under Jaruzelski was determined to crack down on the solidarity. The possibility existed therefore for the situation to be resolved without Soviet intervention.
The feeling by the 1980s that the Soviet Union had overextended itself in its foreign policy and that Communism itself as system was beginning to falter. The priority need was therefore to concentrate resources on the Soviet Union itself rather than on intervening to save Communism abroad.
Source: Binders Blunders and Wars by David C. Gompert, Hans Binnendijk, Bonny Lin