Because "Progressive" is about dismantling the incumbent policies and replacing them with your own policies. The advantages of this is that if you are a political leader, it's something you can hang your hat on to grab a leadership position rather than waiting for the incumbent to retire. But the price of this constant rejection of what your grandparents did is that if you yourself achieve the goal, then your own policies get dismantled by your crusading grandchildren.

Take for example anti-trust, which sought to destroy the power of big trusts in the Teddy Roosevelt era -- and was itself destroyed by the progressive Watergate Democrats in the 70s, about 60 years later. All these anti-trust rules were considered old fashioned, obsolete, and policies of a discredited generation. They were called bigoted and not with the times.

Or the social hygiene movement (https://en.wikipedia.org/wiki/Social_hygiene_movement), another great crusading Progressive cause, that sought to make illegal prostitution, alcohol, public nudity, and other attacks on morals. Few remember that the Women's Temperance movement and the Women's Suffrage movement were part of the same movement, and the progressives of those days really wanted to build a virtuous society, in much the same way that the current crop of progressives is obsessed with virtue, or the 60s crop was also inordinately focused on being virtuous, except for them being virtuous required dismantling the laws of the previous virtue movement. All the moral purity laws from the social hygiene movement were dismantled by other progressives in the 60s-70s.

Or the union rights movement which was triumphant during the FDRs period, but was systematically destroyed primarily by Democrats in the 1970s-1990s (Clinton putting the last nail in the coffin). Unions were viewed as bigoted, old fashioned, and not with the times.

Similarly the civil rights movement of the 60s is being systematically dismantled by the culture warriors of the present era, who view color-blind policies as the legacies of a discredited generation, of bigotry, and not being with the times.

The good news is that if our current pro-racism movement triumphs and the nation re-segregates, there is a good chance that grandchildren of the current generation will themselves dismantle that system as being old-fashioned and not-with-the-times.

In either case, Progressivism needs to be viewed as an act of rebellion first, and the policies are whatever they need to be in order to rally the most people to the moral revolution.