In large part it was because the Communist powers in the relevant states were never really able to consolidate themselves domestically, and were reliant on the Soviets to maintain control. This was evident through the need for Soviet intervention in 1956, 1968, and 1980; without the Red Army coming into play those times its quite possible the Communist authorities would have been overthrown.

Thus it's not surprising that once the threat of Soviet intervention was removed, as was the case in 1989, the opposition increasingly acted to replace the Communists (there is a saying regarding this: something along the lines of "in Poland it took 10 years, in Hungary 10 months, in the GDR ten weeks, in Czechoslovakia it took 10 days." This shows how with each state it became easier for the opposition to take over). Also remember that back in 1945 the Red Army came in not as a welcome liberator, but just a replacement occupier, and the people of the region never forgot that, even with the propaganda that was meant to sway them to the Communist cause.

As well, it should be noted that it wasn't exactly such an easy transition, as the economies of the former Eastern Bloc were in shambles for years afterwards, and many officials from the Communist era retained important positions simply because they were the only ones who had any experience. While it may seem like countries like Poland and Hungary had communist governments and a planned economy one day and a democracy and free market the next, it was a lot more nuanced than that, even if the popular perception suggests otherwise.

Faith in socialism as a way to build a more equitable society (regardless of utopian hopes for the future) was gradually lost from 1970s on, and quite rapidly during 1980s. There are several reasons for that.

First, an era of rapid and extensive industrial development had ended, and intensive high-tech development proved difficult or impossible within socialist systems as they stood. (For that line of argument a good work is "The Rise & Fall of State Socialism" by Lane)

Functioning of propaganda is anything but simple and its effects can't be simplified to people believing or not believing it. Take for example Soviet posters dealing with agricultural collectivisation in 1930s, naive representations of very prosperous mechanised farms, called "socialist realist", in a situation of very low production and lacking agricultural machinery. How could anyone expect people to believe in that propaganda? Well, one of the points in socialist realism is that it is not vulgar realism, but should abstract form real conditions and trends to make a representation more real than reality. In effect, point was not that "collective farms are great" but rather "we are certain that collective farms can and will be great". Same goes for other aspects. Coming back to 1970s and later, the thing is that not even Party elites believed that any more.

For a while still the question of basic social and economic system was not a polite topic, it was restricted to fringes. There is a contemporary parallel. While it is quite normal and expected to criticise current problems, both social and economic, a discussions of underlying economic system is a fringe matter, possible but generally not taken seriously. It might be argued that censuring of discussions on basic social system (socialism vs. capitalism to put it simply) in late socialism even lent a significant credibility to alternative views. After all, people were quite used to reading between the lines of official press.

For an excellent insight into how ideological and propaganda discourse functioned in late socialism I can very highly recommend "Everything was forever, until it was no more" by Yurchak.

By the late 1980s as it were, not even socialist elites believed in socialist project. At the very least they didn't oppose its disillusionment, and often they even helped to bring it about. That would be a key explanation as to why it was let go so easily - there simply was no one left in power willing to defend it.