The computer, the internet, air conditioning, too much time at and going to/from work, both parents working or single parents, needing to spent the remaining time with their kids, economic pressures all have added to this isolation. With less 'free' time, we no longer have the time for church or community activities that brought people together and offered opportunities for and maintain friendships.
The competition among employees at our jobs along with the insecurity situations many face at work kills off the opportunities for friendships there. The cost of housing in decent areas means we have to move away from where we had friends and extended family. The flow and mobility of people, with neighbors moving away and new ones coming in, with different ethnic, religious and economic backgrounds, adds even more to the isolation. In the 1950's, people lived in neighborhoods of similar ethnic and economic situations, like Italian or Irish and Roman Catholic while today, your neighbor may be Asian Indian and Hindu or Islamic.
People also value privacy more today and self-isolate to retain that, thus limiting contact with others. People don't want too much know of them by others and often may have situations that they do want others to know about them.
The range of entertainment available at home, larger homes, other issues mentioned at the beginning of this post, along with the changing attitudes toward alcohol has killed off many bars, social clubs, or situations where alcohol is consumed and contacts with people in the community. In my town, many of the bars are strongly connected with the Polish community and contacts by other Polish immigrants, with few if any bars for non-immigrant members of the community.