Because if you spend enough time on the internet, especially the Western side, you start to hear voices of people who oppose their governments and not enough from those who support it.
Even many who are supportive tend to say that their government is authoritarian.
To elaborate on this, after the French Indochina war in 1954 due to the treaty with France, most of the party members in the South had to move into the North, many of them didn't get to come back South until 1975. The Southern fascist regime also hunted down and killed a lot of our Southern comrades during the war. The lead to a big lost in the Southern branch of the party capability of organization and leadership.
In urban area where the control of the puppet regime is the strongest, they also had a lot of propaganda to brainwash the mass into anti communism. After the war, many of those same suburbanite also saw the biggest lost of quality of life due to the drop in free money from US aid and new sanctions which reinforced those past propaganda. That's why you see a tendency for Southern suburbanite to be skeptical to hostile toward leftist ideology and have a rosier picture for capitalism.