Why were missionary efforts to spread Christianity so much less successful in China than in Spanish America?
This is a good question and there are several important considerations.
First, we need to realize that there were simply more missionaries in Spanish American than in China. The sheer number of people make a difference. Moreover, Europeans settled down in the Americas and along with them they brought their religion. There was a lot of informal missions going on. This was not the case for China. I would say that this is the most important factor.
Second, the Chinese went through a cultural revolution. This meant many things for the nation. When it came to religion there was a massive persecution. Foreign missionaries were kicked out of the country and this fact left the fledgling church with help.
Here is another important perspective, which is very ironic. The church is China is now thriving. Some estimate that there are nearly 100,000,000 Christians in China. To be sure this is a small amount in view of the total population, but Christianity has significantly grown.