The US system is fairly complex. Federal law overrules state law but most issues are decided at the state level. In fact, some issues can not legally be decided at the federal level. Added to that, the population generally dislikes the federal government dictating things. The system is meant to allow people of different regions/beliefs to govern themselves within the framework of their majority without being subjected to what other areas want. Overall it diffuses a lot of tension that would come from a central government setting all of the rules.
Still the federal government can overrule state laws as we see happening with the same sex marriage issue that is working through the country at the moment.