If you want to build a movement you claim represents 99% of the people it’s not a good idea to alienate a big chunk of them by waving the Soviet flag.
Conservatives are generally patriotic. Some are hyper-patriotic. While many people on the left are just as patriotic there is a significant group that is not. Their attitude towards the United States ranges from sneering contempt to outright hostility.
Both the left and right have their fringe elements, but the ones on the right tend to look normal while the ones on the left seem to go out of their way to look and act different. You don’t see a lot of piercings, tattoos and nudity at Tea Party rallies.
People can do what they want, it’s a free country. But if you want to attract mainstream voters to your cause you need to appeal to mainstream values.
People might watch a freak show, but most of them don’t want to join it.