Why were long-term enlistments not a popular idea?

Answer 1

Long-term enlistments proved unpopular because the service-members interests changed. Issues such as marriages, births, deaths, and family problems are distractions, and long-term enlistments provided no opportunity to solve the problem. Also, interests changed; what was important to a young man who enlisted at age 18 may be less important at age 30.

The current (2014) system in the United States calls for enlistment periods of either four or five years, ay which point the servicemember may chose to re-enlist - assuming that branch of the service wants him/her.