Who says that most Chinese don't know about the 89 Rebellion?
I can assure you that it is widely known and understood and the notion that the hundreds of millions of tech savvy mainlanders are incapable of bypassing official restraints, finding out and discussing it in detail, is ludicrous.
gf hits on the right word though "Embarrassment", although in this instance it is the embarresment of the majority in regard to a few naive bookish youngsters being seduced by western notions at a time of radical change and temporary vulnerability (the late 80's) and who made unwise relationships with western journalists and; of course, Intelligence Officers.
The prevailing attitude in China today remains one of optimism and hopes of making money. As the current system is visibly delivering, there is no ground swell of opinion pressing for change.
In fact in the minds of most mainlanders I know, the Tiananmen incident simply serves to link notions of Western Democracy and Human Rights, with, Instability, Chaos, Violence and Destruction, in fact the very things which they increasingly see abroad in Western Countries today and which as a nation, they have already had a belly full of in the not too distant past.