They ran out of money, unfortunately. Energia and their shuttles, Buran and Pichka were very ambitious, and very expensive. The Buran was launched unmanned into space, and orbited the Earth twice, and made a fully automated landing. A successful first flight by any measure. Sadly, the collapse of the Soviet Union meant that the budgets for these programs weren't maintainable financially. The USSR had some very good aerospace engineers. It's a shame, really. You should read all the links others have posted on this thread.