What are the main reasons for the fall of communism in eastern europe?

ECONOMICS.

Communism is a flawed concept, and as such, is doomed to failure.

Well, three reasons come to mind.
1. The relative overemphasis of military support vs. economic needs, particularly in Russia proper but also endemic to the Warsaw Bloc as a whole.
2. The relative economic collapse of the Soviet collectivist system proper.
3. The military/economic impositions placed on the "Warsaw Pact" nations by the USSR, in contrast to the Marshall Plan and similar policies implemented shortly after WW2.
4. Chronic and systematic central planning in a command economy (economy by plan) contrasted with (economy by the people) where self-interest dominates the economic course of events vs. necessarily state-interests.
5. Lack of infrastructure and economic reinvestment - this is a problem for every major economy but in the old Warsaw bloc states, deteriorating conditions did not allow for the agressive reinvestment over time.

An EXCELENT book on the matter is Paul Kennedy's "The Rise and Fall of the Great Powers".

Solidarity in Poland
The Polish communists dropped efforts to impose Soviet-style collectivization on the peasants and to break the Catholic church.
The Polish economy suffered greatly because of poor leadership and the world depression of the 1970s.
The "Polish miracle" occurred when the economic crisis became a spiritual crisis as well.
Pope John Paul II, former archbishop of Cracow, called attention to the rights of all people.
Strikes in August 1980 led to revolutionary demands, which were accepted by the government in the Gdansk Agreement.
Lech Walesa led the new democratic trade union movement called Solidarity. Its demands were for industrial, political, and economic rights.
Solidarity had massive support and a sophisticated organization.
It stopped short of directly challenging the communist monopoly of power.
When Solidarity lost its cohesiveness, the Polish communist leadership under Jaruzelski smashed the movement (1981) and imposed martial law.
After 1981, Solidarity went underground and fought on with great popular support.
Polish cultural and intellectual life remained vigorous despite the repression.
Gorbachev's reforms in the Soviet Union
A new era of fundamental change began under Gorbachev in 1985.
By 1982, economic decline was worsened by mass apathy and lack of personal initiative.
Andropov tried to reinvigorate the old system but with no success.
Gorbachev set forth a series of reforms to restructure the economy (perestroika), centering on a freer market economy, but the economy stalled midway between central planning and free-market mechanisms.
He instituted glasnost, or openness in society and politics, leading to much more freedom of speech.
Democratization of the Soviet state was begun; free elections were held in 1989 for the first time since 1917; meetings of Congress were televised.
Democratization encouraged demands for autonomy by non-Russian minorities.
Gorbachev withdrew troops from Afghanistan and encouraged reform in eastern Europe, repudiating the Brezhnev Doctrine.
The revolutions of 1989
Gorbachev's plan to reform communism snowballed out of control
A series of anti-Communist revolts spread across eastern Europe.
Gorbachev's reform plans in the Soviet Union backfired as anti-Communist revolts took hold.
The collapse of communism in eastern Europe
Gorbachev's plan to reform communism snowballed out of control. [NOTE: THIS COMMENT REPEATS THAT OF ITEM II A ABOVE. OK ANYWAY?]
In Poland, Solidarity was again legalized and won overwhelmingly in free elections.
Gorbachev refused to send Soviet troops to keep Polish communists in power.
Many radical political and economic reforms were instituted.
In Hungary, popular resistance and communist liberation ended one-party rule and brought free elections in 1990.
A multiparty democracy was established.
Borders between Hungary and East Germany were opened.
Growing economic dislocation brought revolution in East Germany.
The Berlin Wall was opened.
Communist leaders were swept out of power.
The people of Czechoslovakia ousted the communist bosses in 1989.
Only in Romania was the revolution violent and bloody.
Ceausescu was executed.
Romania's political prospects remained uncertain.
The disintegration of the Soviet Union
In 1990, the moderate Gorbachev was in between hard-line communists and revolutionary democrats and anti-Communists.
Groups (i.e., in Lithuania and the Caucasus) still within the Soviet Union were challenging Soviet (Great Russian) control.
In February 1990, the democrats and anti-Communists won local elections; Gorbachev's new constitution abolished the Communist party's monopoly of power.
Gorbachev was elected president of the Soviet Union.
The radical Boris Yeltsin was elected leader of parliament while Gorbachev tried to keep the Soviet Union intact.
In an attempted coup, hard-liners kidnapped Gorbachev; Yeltsin saved the government and brought about more reform.
By declaring Russia independent from the Soviet Union, Yeltsin caused the dismemberment of the Soviet Empire.
Gorbachev's job ceased to exist.
Russia now concentrated on building a strong Russia.
German unification and the end of the cold war
The death of communism in East Germany reopened the question of German unification.
East German reform communists had feared unification and looked for a "third way."
This idea failed because half the population fled and because West German Kohl offered a generous economic plan to bankrupt East Germany.
The key to unification was Gorbachev's approval (1990) in exchange for aid; Germany was officially unified in 1993.
The Paris Accord of 1990 brought twenty-two European countries to agree to arms reductions and to affirm existing borders.
The Americans and the Soviets followed with a significant reduction in nuclear weapons.
With Russia in decline, only the United States was left as a world superpower.