I think you mean "where" not "why". From the 3rd Article of the Constitution. The Constitution only establishes the U.S. Supreme Court, but it gives the Congress the power to establish lessor federal courts. TheJudiciary Act of 1789 setup the lessor courts and has been amended many times since 1789.
Congress has the power to create new federal courts.
Congress
The authority to set up a system of federal courts was granted to Congress by the U.S. Constitution. Article III of the Constitution establishes the judicial branch and allows Congress to create inferior courts under the Supreme Court. This provision enables Congress to determine the structure and jurisdiction of the federal court system.
Constitutional Courts - mandated by the constitution Legislative Courts
They created it in 2010
Congress has the power to create lower federal courts, therefore the Legislative branch has the power to create these courts.
The Constitution gives Congress the judicial powers to create all of the federal courts below the Supreme Court and to structure the federal judiciary. Congress also has the power to define federal crimes and set punishment for violators of federal law.
Congress is vested with the authority to create courts "inferior" to the US Supreme Court in both Article I and Article III of the US Constitution.
They have the power to create new federal courts.
Congress has the power to create new federal courts
Under Articles I and III of the Constitution, Congresshas sole authority to create federal courts. They established the US Supreme Court (mandated by the Constitution) and first federal courts in the Judiciary Act of 1789.
It is true that the United States Constitution only explicitly and specifically establishes the Supreme Court of the United States. However, it also specifies that the Congress should create lower federal courts, which Congress did with the Judiciary Act of 1789, and as amended. There are also "Article I courts", created by the Congress, that are technically part of the federal executive branch and not part of the federal judicial branch.