Whatever, in your teenage years and early 20s, looking back, you forget the anxieties and negatives. From Zagreb to Gdansk, remind folks of those years and their eyes glaze over as they remember student life, loves, parties, concerts. Sure, some do hark back - but if you forced them to give up their passports to the west, restricted their access to low-quality Soviet-bloc goods and (in some cases) rationed their petrol to 8 -10 litres per month, they'd very soon be complaining and wanting change.
My impression (open to correction from those with more knowledge of the countries concerned) is that much of the Ostalgie is found among older people, who suddenly went from having a job for life (however dull and badly paid) to having to compete for jobs in a changed world with high unemployment and different skill requirements.
Also, I expect that those involved in protesting against or bringing down the system might look back nostalgically to that.