It seems vital to Russian interest and they kept Austria succesfully out of NATO earlier. Was it that Moscow didn't have enough leverage? Did they even try? Was it too chaotic?
This is controversial and heavily disputed but the Soviets did receive informal guarantees that NATO would not move east of Germany in 1990 when German reunification was being negotiated. Secretary of state James Baker offered assurances in Moscow that NATO would not move "one inch" eastwards from its present position. West Germany foreign secretary Hans-Dietrich Genscher hinted that NATO would not even be moved eastwards to cover the former territory of the GDR after reunification.
However, Gorbachev never received formal guarantees that NATO would not move to encompass former Warsaw Pact countries, the negotiating position of the USSR was weak in 1990 due to economic chaos in the country, and West Germany had (informally) sent a significant amount of financial aid to the USSR in exchange for reunification. West Germany paid the USSR perhaps the equivalent of 40-70 billion Deutchmarks in 2016 terms, which is a considerable amount.
Had Gorbachev wanted to, he probably could have extracted a far larger sum from West Germany as price for reunification and perhaps formal guarantees on NATO expansion as well. But a combination of domestic economic troubles, a Soviet political leadership losing control at home, and simple bad negotiation on the part of Gorbachev explains why only informal guarantees were given.
Source: German Reunification: A Multinational History edited by Frédéric Bozo, Andreas Rödder, Mary Elise Sarotte