The reason is quite simple and would have been apparent if Trump had taken the time to read his own presidential memorandum as it explains why the proposed cross-border pipeline is not subject to Trump's U.S.-made steel promise.
The memorandum states that the rule would only apply to new pipelines or those under repair. Since this one is already currently under construction, the steel is already literally sitting there and none need be manufactured.
In fact the memorandum does not name Keystone XL or Dakota Access and does not even mandate all US content in future projects. Instead it only says materials and equipment "should" be made in the US “to the maximum extent possible and to the extent permitted by law”.
Yet in a speech at CPAC last month when he claimed that he personally insisted on the buy-America idea himself...
"This took place while I was getting ready to sign. I said, 'Who makes the pipes for the pipeline?' Well, sir, it comes from all over the world, isn't that wonderful? I said, 'Nope, it comes from the United States or we're not building one.' American steel. If they want a pipeline in the United States, they're going to use pipe that's made in the United States, do we agree?"
Trump then lied again during a Feb. 23 meeting with manufacturing CEOs at the White House, Trump told U.S. Steel Corp chief executive Mario Longhi that...
“the pipe is coming from the U.S.” for the Keystone project, as well as Energy Transfer Partners’s Dakota Access pipeline. We put you heavy into the pipeline business because we approved, as you know, the Keystone Pipeline and Dakota. But they have to buy — meaning, steel, so I’ll say U.S. Steel — but steel made in this country and pipelines made in this country.”
It was only after people noticed that four days later, when Trump addressing a joint session of Congress, that he had walked his previous remarks back noting that he had issued a directive requiring “new” American pipelines to be made with American steel...
"We have cleared the way for the construction of the Keystone and Dakota Access Pipelines, thereby creating tens of thousands of jobs. And I’ve issued a new directive that new American pipelines be made with American steel."
It was three days after that when a White House spokeswoman told reporters traveling aboard Air Force One that the Keystone XL pipeline was exempt from the order requiring American pipelines be built with American steel because it was not a “new” pipeline.
Trump didn't even have the guts to come out and correct the record himself.
As it turns out the company responsible for building Keystone, TransCanada, has stated in a press release that the majority of the steel would come from North America, which includes Canada and Mexico with the remainder coming from Italy and India.