The main social force that makes it harder to extended families to function is the force of our mobile society. In our society today, it is very common for members of families to live far apart from one another as family members marry people from other communities and/or move to find work. This makes it very difficult to maintain tight bonds between members of extended families.
One force that can work in favor of extended families (when they live close enough to one another) is the move towards dual income families. When such a family lives near enough to older family members (typically parents), the older generation is often needed more than it was when a parent always stayed at home. Grandparents are often used as providers of day care and other such services for busy families. This ties extended families together more tightly than they might have been a few decades ago when dual income families were rarer.