Why are insurance contracts of 6 months to a year?

Most companies offer policies six to twelve months. At the end of the term of the policy, either party (or both) may offer to continue the contract the way it is, choose or change the premiums (either up or down).

However
 
Unless the company believes that too great a risk usually be offered to renew after the period is over, usually nothing happens, except to continue payment of premiums and the company continues to insure you although you may see a change in the premium.

6 and 12 month policies
 
For those companies that offer both six and twelve month policies, the twelve month policies are usually a little higher because the law in most states, the insurance company can not choose or change the policy after been in force for a specified period (usually sixty days) and the insurance company is accepting the risk (in terms of insurance means that the insured property) for a longer time period.