According to Ron Paul:
The United States Constitution gives Congress the authority to regulate the time, place, and manner of federal elections. Thus, ballot access is one of the few areas where Congress has explicit constitutional authority to establish national standards.
That authority is something they don’t mind using, especially when it comes to monopolizing the political system by creating obstacles for third parties. I guess the current obstacles weren’t stringent enough, so they plan on :
On February 1, congressional Democrats, led by Rep. Obey of Wisconsin, introduced a bill, H.R. 4694, that would end viable, third-party competition in races for the U.S. House of Representatives.
Perhaps they feel threatened?
“The Republican and Democratic parties exist to maintain power for their own benefit. The Libertarian Party exists to grasp power for the benefit of the nation,” stated Shane Cory, chief of staff for the Libertarian Party. “American voters are waking up to this reality, and as they do, the two parties are trying everything within their power to shut us down.”
More info here.