I am aware of that. I am also aware of underground churches and missionary activity (not LDS) in countries where preaching/teaching the gospel is not allowed, in fact it can get you killed. As a matter of fact, I work extensively with with a ministry that helps Christians that are facing persecution in those countries: Voice of the Martyrs (persecution.org).
But the question I asked was WHY do the LDS MOSTLY do their missionary work in countries (like the US, Canada, South America, Europe) where the majority of people have already heard of Christ? And why do they seek out those that are at least nominally Christian for conversion to the LDS church?
Well, if you don’t like my answers, you should probably ask President Monson since he assigns missionaries. I do not have that calling.