House and Senate Journals, Series I, 1789-1817
The original Journals of the Congress for the first 14 Congresses were printed in small numbers by non-government printers on contract. In the years after the burning of the Capitol by the British in the War of 1812, copies of those House and Senate Journals gradually became increasingly rare. So Congress authorized Gales & Seaton to reprint the Senate Journals for the first 14 Congresses in five volumes in 1820 and the House Journals in nine volumes in 1826.
The Readex digital House and Senate Journals, Series I of the first fourteen Congresses reproduces the original texts of these important works, not the reprints containing some textual differences, deliberate or inadvertent, from the originals.
In answer to the question of why these journals are not in the U.S. Congressional Serial Set or the American State Papers, it need only be pointed out that they antedate the Serial Set, which begins in 1817, and that they were not again reprinted in the American State Papers because Congress had only relatively recently authorized their reprinting as stated above.