It seems that most of the people i know have lost many of their back teeth and less of the number of front teeth. why does it happen this way?

Anatomy: Back teeth, because of their shapes, are more susceptible to decay. It's also harder to care for supporting tissues. Add to that not bruising 2x/day for 2 full minutes by the clock, not flossing, not having check-ups 2x/year, eating hard foods like ice, high sugar diet, not replacing missing teeth immediately, etc. comes down to see your Dentist regularly and follow advice to the letter.

Location: Most people don't brush or floss as they should. It's easy to see the anterior teeth, but people often don't put in the effort to clean the posterior teeth properly. In addition, people often chose to extract a tooth than spend the money to restore it... posterior teeth are not as visible as the anterior teeth.