Ask yourself this question:
Where did all the Georgists go? Many if not most of them "made progress" onto the next logical step in the goodness of wealth redistribution and all forms of nationalization - not just land nationalization. Socialism. They became socialists. That's where the Georgists went. "Making progress".
There are way too many high-profile early progressives who were highly driven Georgists(or directly related to them) - for any other answer to be accurate. Bernard Shaw is an easy high profile example. He came to activism via George. Before long, he was reading Marx, and he finally went so far as to advocate for killing people in gas chambers.
I'm absolutely not saying that George is responsible for what Shaw became - Shaw "made progress" many years before that. He left Georgism by the side of the road for his new ideology. What I am saying, is that this word "progress"...... "progress" is far too often ignored and/or misunderstood by WAY too many conservatives in understanding how these people get to where they got. "Now" is killing us.
When you start to examine progressivism as a timeline, and if you truely want to start at the beginning, there's Henry George. And I'm not talking about disparate ideologies which came from far off lands, which while important, are disparate. They're minor roots of the tree. Henry George is not only a major root, he's a part of the tree trunk.
You mentioned Edward Bellamy. How do you think people were willing to accept what Bellamy was peddling? They had had a decade of the Georgist teet to suckle themselves on. Looking backward was published in 1889. Progress and Poverty was published exactly 10 years earlier. 1879. I don't care what anybody says. That's not a coincidence. I don't believe in coincidences when it comes to progressivism, that's foolish. One book clearly leads directly to the other. All these people back then who were the readers, they all "made progress". That's where the Georgists went. They "made progress".
Henry George was a critic of the capitalism of his day, and also a critic of state socialism. If you're looking for socialism, George and his ideas about land and the single tax could serve as a stepping stone to socialism.
If you weren't headed in that direction, you didn't have to go there. Libertarians like Frank Chodorov and Albert Jay Nock were also admirers and (for a time at least) followers of Henry George. More here.