The answer is quite simple and tragic. They got invaded and massacred by the Soviet Russian army. Indeed, the government of new first secretary Alexander Dubcek had debated, decided and started implementing a series of democratic reforms that still kept the country socialist but democratic; the best of both worlds in a sense. Parts of the economy were decentralized and somewhat liberalized. Liberalization also applied to free speech, the capacity of the media to criticize and report on government activities and above all freedom to travel outside of the country.
The Soviet Union sent 650,000 troops armed with the most sophisticated military material of the time to crush what is now known as the Prague Spring. The country resisted for 8 months, especially through acts of civilian resistance and sabotage. Unfortunately, the Soviets prevailed and thousands of people fled the country, never to come back. The effect was devastating in terms of public image for the Soviet regime and most left-wing groups in western Europe started advocating for complete ideological separation from the Soviet Union and the emergence of an European form of Communism or Socialism.