Communism is a flawed ideology that will never last for very long. The only reason it has lasted as long as it has in some Soviet countries is through ennui and apathy. As younger generations reach maturity and gain a greater percentage of control, communism will fail in those countries too.
Communism should only become more mainstream if the will of the majority of the people wish it so. The majority of the world does not live under Communism, so proclaiming it as being mainstream would not be possible in this sense. Long-lasting, centuries-old tribal leadership exists in many third-world countries but we do not claim it as mainstream just because it has been around this long. Nor should we.
Communism is already mainstream enough, it does not need to spread. It has also been mostly a failure, because everyone does not have the same beliefs, and work ethic for it to really work. Countries that have survived with a Communist government, have had to inject certain other economic beliefs to succeed. China is a prime example of this.
So far there has not been a communist country that is reliable, good to its citizens and in good shape. Communism as a theory may work well, but in practice it has yet to become something that works for the best of all citizens, no matter that some countries have been communist for decades.
Communism is a great idea on paper but in practice it is massively flawed, whether it has remained relatively stable in a few select areas or not. For a majority of the planet, the cultures are not once that have raised people that would enable communism to function how it is designed to.