Why was the US neutral until 1917?

Until 1917 The USA was neutral and were not involved in combat in WW1.

One reason was that they were isolationist and did not see any reason to become involved in the war. Furthermore, their country had been created with the idea of maintaining peace within the country and providing a "safe-haven" and escape from the war in Europe. Therefore, to follow their policy and original foundations of the country, they decided to continue their neutrality.

Secondly, as the US were able to benefit from the war by taking over both German and British markets in South America, many Businessmen were satisfied by their neutrality. The US were able to do this because countries in Europe were focused on combat and supplying their armies.  As a result, they were able to make money through trading to these foreign markets, but also by setting up loans to countries involved in the conflict that would be repaid with interest once the war had ended.

Another reason was that much of the US was isolationist. This was shown by Woodrow Wilson's policy of neutrality and the fact that he was re-elected based on his commitment to peace. Furthermore, there were a number of German and Irish-American communities with anti-British views, so they opposed the US entering the war in favour of the British. Therefore, as there were not any US security problems as a result of the war at the time, there was no reason for the US to get involved.