Because eternal salvation is a lot more important than 80-or-so years on earth.
This mindset is why traditional Christianity is distasteful to me. It places evangelism over compassion for your fellow man, and it paints God as an asshole; the former is dehumanizing, the latter frankly blasphemous.
I understand where you’re coming from, but why is life more important than knowing truth? Not that all possible precautions shouldn’t be taken to prevent the loss of life before spreading the truth. Christianity rejects the modern premise that truth is either unknowable or unimportant, and because Christians believe truth to be inextricably tied to eternal life, knowing and living (or dying) according to the truth is the greatest good. And consequently, a short life with knowledge of the truth is better than a long life in ignorance of it.
It is quite possible, however, that because these people have never known about God, they could never reject him, and wouldn’t condemn themselves to hell because of it, but that’s a risk that many Christians wouldn’t want to take. Not to mention that one’s capacity for happiness in heaven is dependent on the way one increases that capacity by growing in one’s relationship with God on earth.
But I certainly understand the dilemma, and haven’t entirely resolved it in my mind either.