Here is a simple thought experiment to lead us through the idea of "the unreached."
1. We know that there are, maximum, about 2.5 billion Christians. In a world with 7.7 billion people, Christians therefore make up about one-third of the world.
2. We know that these Christians are "unequally distributed" - that there are many Christians in some countries, where they form a large percentage of the population, and very few Christians in other countries. For example, at the time that Operation World was last printed, there were thought to be fewer than 1,000 Christians in Afghanistan, excluding the expatriates who come and go with the vagaries of politics.
3. In any given population, if the number of Christians is very small, then obviously the number of non-Christians is very large, and forms a very large percentage.
4. The world speaks thousands of languages. Christian materials are not available in all of them. Further, the distribution of Christian material is skewed toward Christian populations ("markets"). Coca-Cola isn't sold everywhere, and Christianity isn't distributed everywhere, in every language.
5. Many governments and even some populations are largely opposed to the spread of religions other than those which the nation approves of (for example, Hinduism in India, or Islam in Saudi Arabia). They therefore actively seek to stop the distribution of Gospel materials and the presence of Gospel witnesses.
6. With large populations that have small numbers of Christians among them, who speak a language with very little Christian material, who do not represent a significant market to attract Christian investment in literature and media, and who are in a nation led by a government hostile to Christianity, it is clear that some (possibly many) segments of the population will be "out of reach" of the Gospel: that is, a single individual could be born, live, marry, work and die without having had a Gospel influence in his or her life.
7. If someone doesn't buy Coca-Cola because there are no Coca-Cola distributors in their neighborhood, it's not that they "dislike Coke" or "prefer Pepsi" - rather, the reason for their decision not to buy Coca-Cola is that there is no Coca-Cola to buy. Ditto for the Gospel: people in these situations are not resistant. They do not choose Jesus because they have never been presented with the choice.
8. If there are no Christians, or insufficient Christians, in a non-Christian population to bring the Gospel everywhere, then the Gospel must come from other places where resources are more available. This is why we go on mission. As Piper famously said, "mission exists because worship doesn't."
So, which should get priority - the non-Christians in your neighborhood, or the non-Christians in a "no-Gospel-access" neighborhood a world away?
Given how much wealth is held by Christians and the church - why can't both get equal priority?