With certainty, I can only speak about Poland. I guess that it was similar in Czechoslovakia and USSR, but probably the popularity came later.
The boom came in the 60ties, culminated in the 70ties and never faded entirely. All the publishing houses in Poland were state-owned and strictly controlled, so (for sure) there was a political decision to translate numerous authors from Latin America. Maybe it was a part of larger politics - countries from the soviet bloc were strengthening relations with South America to counterbalance the influence of the USA. Anyway, it was a huge success. Magical realism was entirely different from whatever you could find in Poland at that time. But on the other hands, Latin American societies were in many aspects similar to ours. Semi-peripheral countries, in the shadow of the superpower, struggling with poverty, authoritarian rule, catholic tradition. For some, these books were windows to the exotic world behind the iron curtain. There was even a dedicated publishing series: Proza iberoamerykańska, where more than a hundred titles were published (see here:
http://lubimyczytac.pl/seria/62/proza-iberoamerykanska?widok=lista&kolejnosc=data-wydania). Even today, writers from Latin America are very popular and their books often become bestsellers.
Some notable Polish writers, esp. Olga Tokarczuk and Paweł Huelle, are strongly influenced by Latin American literature, and their prose is sometimes called "Polish magical realism".