The majority of missionaries in Asia built schools and taught European languages and Western science, administered medicines and vaccines, performed surgeries, and facilitated many other valuable services.
Last Reason: Missionaries involved themselves in questionable schemes.
A few Christian missionaries involved themselves in questionable schemes, knowing or unknowingly. Some pocketed money from their parishes at home meant to benefit Asians and vacationed instead. Some refused to administer desperately needed supplies and medicine unless the person became Christian, otherwise known as forced conversion. Some missionaries interpreted for Opium smugglers paving the way for the drug to penetrate deeper into the interior of China. Some acted as missionaries while secretly working for European governments aimed at undermining Asian states and ultimately at colonization.
Asians had reasons for distrusting European and American missionaries.
Take a look at this Al Jazeera report about missionaries working in Thailand in 2009.