The US sets an annual quota for global refugees, as well as regional quotas. In 2015, the US anticipated taking only 33,000 refugees from the Middle East and North Africa — and that included a deliberate effort to take thousands of refugees from Iraq.
The US can exceed a regional quota, but it can’t turn on a dime. Resettling refugees is a lengthy process, one that usually takes multiple years. It starts with deploying processing personnel and resources on the ground to find and vet refugees for resettlement, and continues with the long time it takes to process an individual resettlement application: usually 18 months to two years.
[21.10.15 update: where do they all go? This NYT interactive map shows.]