They were not sold for two reasons. First, as /u/mihato said, at first only Western countries could produce them in large quantities - it was simply too expensive for an average Eastern bloc guy.
In my country, Hungary for example one could only buy jeans from the black market for as much as 1500 forints, which was then a half month salary of an average worker. These jeans reached the black market mainly from Yugoslavia where they were freely sold. Also, the Communist party simply didn't let Western consumer goods in, because they feared if the people spent their money on expensive stuffs, they would realize how low their wages really are.
The other reason was quite obvious: it came from the West and, just like with Coca Cola, it symbolized Western culture and Western lifestyle which was officially looked down (not by the society though, and it's very important!). It didn't matter that jeans were absolutely working class thing.
By the late 1960s and early 1970s however the system started to soften (at least in Hungary, but for example in Romania it was quite the opposite), and not only Western music was let in, but also clothes and other things. What's more, in 1968 Coca Cola signed a contract with a Hungarian factory and for the first time in the Communist bloc, coke was not only produced by also allowed to be sold in Hungary!
Let's get back to jeans. By the early 70's, Hungarian textile companies started to experiment with denim and in a few years they reached quite good quality so they started to produce and export them in large quanitites. Also, in 1977 Levi Strauss and the Marcali Textile Factory, later also the Kiskunhalas Textile Factory (Hungary had a huge textile industry before the cheap Chinese goods ruined them) signed contracts to produce jeans which lasted until 2008 as far as I remember.
TL;DR: before the 60's they weren't sold because of economic and political reason, after that they were let in and domestic production began