In countries like Papua New Guinea the JWs do also target public venues such as the markets (certainly during my time there, this was the case). Further, it always seemed to me that they met with more success doing that than futilely knocking on people's doors.
Not to mention, too, that for many of the population, "home" is a hovel in some crime-ridden squatter settlement. Not sure how you would go about knocking on somebody's door when there isn't one!
The JWs did "witness" within the squatter settlements, but even there they approached people only in the public places of those areas (such as the markets). To have approached somebody's dwelling within those settlements would have been an almost certain way of receiving physical attack - usually by some wild character wielding a bush knife!