What they probably mean is "The distributors we buy our products from don't carry it."
We have our own attitudes toward the value of retail. RPGs are bad bang-for-buck when it comes to retail for a lot of reasons I'm sure other people can explain.
But the result is, most game stores do not want to stock RPGs. That makes sense to us! RPGs are a terrible value proposition for a hobby store!
But stores don't like it when someone walks in, asks about a product, and then leave without spending any money. So what they tend to do is order RPG books as a hedge against exactly that scenario.
They have no interest in actively selling RPGs, but they don't like it when someone walks in, asks about an RPG, and leaves because it's not in stock. So the store orders...three copies.
"Three copies please" is the "We have no interest in selling your product, we just want to make sure there's one on the shelf in case a rando walks in and asks." Yep, we get it. Makes total sense. But since you're not interested in selling our product (i.e. you do not believe in it), we aren't interested in selling to you! It takes time which is money to manage those accounts and we got better things to do.
Those stores can only get our products by buying from a distributor. But we do not LIKE selling to distributors because they want us to send them our products and then nothing happens for six months and then MAYBE we get paid.

So we tell distributors "Sure we'll sell you some books, but you pay for them first, then we ship them."
"What?! But that's not how we do business!" Well, that's fine. How you run your business is...your business. But this is how we run our business. We're not a charity. We need to get paid for our work.
Well that still leaves a handful of distributors, so you sometimes see our books in game stores!
Then there are the retailers who believe. They see Draw Steel and think "I can sell that." Meaning, someone walks in and the guy behind the counter says "Hey have you heard of Draw Steel? I think you'd like it!"
That retailer is going to order a lot more than 3 books and we will sell those books to them, direct, and we pay for shipping. But that means they need to meet a minimum order.
We think there's about 20 retailers in America this is true of.