Almost every distinctive position in the Pittsburgh Platform has since been revised by the movement that adopted it, and the revisions are documented, which makes this an unusually clean case of a religious body reversing itself in public.
The first break came from outside. The rejection of the dietary laws and of the hope of return was more than traditionalist rabbis in America would accept, and the reaction to Pittsburgh helped consolidate the movement that became Conservative Judaism, which founded its own seminary in New York within a year.
The rest came from inside, and slowly. The Columbus Platform of 1937 restored a positive statement about the Jewish people and about the rebuilding of Palestine as a Jewish homeland, which 1885 had explicitly disowned. The Centenary Perspective of 1976 went further on peoplehood and on the legitimacy of diverse practice. The Statement of Principles adopted in Pittsburgh in 1999 affirmed the study of Hebrew and the practice of mitzvot that individual Jews find meaningful, in a tone that reads as a deliberate answer to the document of the same city.
The forces behind the reversal are not obscure. Zionism, the destruction of European Jewry, the founding of Israel and a large eastern European immigration that did not share the assumptions of 1885 all pushed the same way.
The platform remains the clearest statement Reform Judaism ever made of what it thought it was, which is worth reading precisely because the movement no longer holds it.