Why hasn't the pope consecrated Russia to the Immaculate Heart?
The short answer
He has. On March 25, 1984, John Paul II, united with the world's bishops, consecrated the world, and especially those peoples most in need of it, to the Immaculate Heart. Russia was named indirectly, and Sr. Lucia said Our Lady's request had been fulfilled.
The consecration took place on March 25, 1984. Pope John Paul II, in union with the bishops of the world, consecrated the world to the Immaculate Heart of Mary, and "in a special way" those individuals and nations most in need of it.
Russia was named indirectly, probably for political and ecumenical reasons. But Sr. Lucia dos Santos, the last surviving Fatima seer, assured the papal nuncio to Portugal that Our Lady's request had been fulfilled. EWTN has published a timeline of the consecration and related events.