For one thing, the problem is your tanks and troops have to be ready all around the country - the protesting people are moving across the state quickly. One day there's a protest in Prague, second day it's in Brno - but you can't move your 300k troops and 5000 tanks from Prague to Brno in a day. And then the next day it's Ostrava and you have to do it again. Then an incident happens and that provokes a 10x bigger protest in Prague, Brno and Ostrava at the same time. That's impossible. You need much, much more troops and tanks to handle this scale of rebellion - and the requested air support that never came. And your tanks will never make people go back to work, anyways.
(I'm discussing Gorbachev, not GDR specifically)