See also: Tiny Houses
Yeah, but you don't give obesity education to starving people. It's even in the word "declutter": you can't exactly do it if you don't have clutter.
There's a happy medium between deprivation and luxury, and generally, what makes people happy is farther from luxury than most people think. I've met people who didn't know where there next meal was coming from, and I've listened to friends who make triple the US median rant about how they don't understand how anyone could live on less than they do. Neither group was happy, but I wouldn't dare preach the benefits of anti-materialism to the former group, because they clearly need more material goods than they have now. And yes, it's a bit of a luxury, in a different sense of the word, to be able to opt out of luxury, but the narrative was never "live like your poor" so much as "live like you don't need stuff that you don't actually need."
Although, I admit it probably sucks to hear the praises of "doing more with less" when you barely even have enough to survive. It's a taunting reminder that not everyone has it as bad as you, that some people have so much, even, that they ought to get rid of their excess. But that doesn't mean that everyone needs to do more with less than they have right now, it just means that the target of the message, like so many others, is the bourgeois upper middle class, not the working class who are barely scraping by.
This really isn't true of all people that line up for Black Friday sales, though. I know rich people with houses full of things they don't need that do just that, and buy a bunch of shit they never would have considered buying if it weren't on sale. If you think about it, that's the point of sales, to sell more things at lower profit and make up in volume. And hell, most of the poorer people I know can't stake out those sales, because they couldn't get off of work, because it was Black Friday.
And although I understand it, I think it's tragic when a poor person living paycheck to paycheck saves up and buys a luxury item that's on sale*. It would be fundamentally more rational for them to save it for the next flat tire or unexpected doctor visit or whatever. This is a more insidious form of materialism: that new TV might make them happier than the old one... for a little while at least. That doesn't make it a good idea to buy the TV, but it's much easier to empathize with than a rich person buying that TV.