Why does ideology force Berger to diss their work and minimize how popular and well-liked they were? Because they showed ordinary people enjoying themselves in the first mass society in the world in ordinary everyday settings. But Marxism says ordinary everyday society is oppressive and the people ground down by a brutal capitalist elite. He can't get himself to understand that Western European everyday life of the late nineteenth century was the best life ever for ordinary people anywhere in the world or anywhere in history, and the fact that they were out happily enjoying it in a civilized urban way was a new thing in the world, and depicting that was an important advance in art.
I don't necessarily agree that in a vacuum you can really classify that as the best period ever in an objective way unless you have specific metrics, but also you don't really have to. Certainly it would've been an interesting time to be alive. I am actually intrigued in learning more about that period now I will say.
But I think the reason that Berger focuses on the critical class analysis of art is in the title. He's mainly focused on unpacking the way "fine art" gets valued and how/who those ideas of what is desirable, beautiful, etc were shaped. Then he ties that to the way the modern ruling class of his time used art to shape what is valued and desired with the tech/economic advancements at the time.
So focusing on the beauty of specific pieces wasn't really the focus of his work, as I imagine plenty of other contemporaries were already covering that.
As an artist I also don't think that it necessarily takes away from the art to acknowledge that what I make is shaped by my need to also make money and the markets they exist in affect that. I think it can be equally true that the impressionists were capturing nostalgic scenes of everyday life in a novel way that that people would want to buy, but that also was a product of people similarly working within what would sell in a market (which would be an advancement on previous conditions where people could even choose to pursue that kind of life which is revolutionary for the time).
He even acknowledges how his book is only the start of an analysis and it would be neat to see someone pick up this work not just from the perspective of the ruling class that commissioned the art but also the artists who made it and their stories. Obviously a lot of that gets lost with history though so that's probably harder to do.
Berger has a different focus specifically on critique because it's easy to look at the surface level of an individual piece without understanding who that art is made for and why that art is somehow valued above other art.
Especially as someone who didn't commission the piece to sit in my home and has no real relationship to these art pieces other than they're old, I find it helpful to understand how it was initially made and then how modern society treats it in continuing a value system that places certain art above others as more "worthy".
Frankly I'd argue he's critiquing the value system that would just off handedly say "virtual art has gone downhill" just because now it's more accessible and easy to create than ever