Russia is no longer socialist, but both now and back then it was sanctioned like crazy. The US Empire physically cannot blockade it like they can Cuba. Cuba also does focus on exporting revolution, at least decades ago, such as in South Africa. As for China, it's because the US Empire is dependent on it.

China's rapidly improving productive forces and cheap labor ended up being an irresistable match for US financial capital, even though the CPC maintained full sovereignty. This is in stark contrast to how the global north traditionally acts imperialistically, because it relies on financial and millitant dominance of the global south. This is why there is a "love/hate" relationship between the US Empire and PRC, the US wants more freedom for capital movement while the CPC is maintaining dominance.

Fast-forward to today, and the benefits of the CPC's gamble are paying off. The US Empire is de-industrializing, while China is a productive super-power. The CPC has managed to maintain full control, and while there are neoliberals in China pushing for more liberalization now, the path to exerting more socialization is also open, and the economy is still socialist. It is the job of the CPC to continue building up the productive forces, while gradually winning back more of the benefits the working class enjoyed under the previous era, developing to higher and higher stages of socialism.

In doing this, China has presented itself to the global south as an alternative to the unequal exchange the global north does with the global south, which is accelerating the development of the global south. China is taking a more indirect method of undermining global imperialism than, say, the USSR, but its been remarkably effective at uplifting the global working classes, especially in China but also in the global south.