It's not often people give a hoot about physical pre-order incentives, but Fallout 4's Pip-Boy edition is a huge exception. It's easy to understand: Pip-Boy's are great, and this one even threatened to be useful, compatible as it was with smartphones. Such was the demand that if you haven't secured one already then you've got no chance, unless from a reseller.
It's not because Bethesda is stingy or mean, but rather because they simply can't. According to Pete Hines in an interview with Gamespot, factories can no longer make them.
"We reached a point where we'd go back to the factories and they were like, 'guys, this is it, sorry. This is as long as we can run the lines and as many of them as we can make.'"
"They're being made today, it's not like they're done and sitting in a warehouse. [The factories would tell us,] this is what the yields say. I mean, we don't make [the Pip-Boy Editions], and we'd go back to [the factories] and say, 'Demand for this is insane, we've got to make more.' And they'd move other projects off or shift stuff to other factories and it just came to [them telling Bethesda], 'Final answer: sorry, this is as many as we can make.' And we sold every single one of those that we could."
So there you go: video game merch doesn't just magically appear in some marketing department storeroom, it needs to be made. Sometimes there's a limit to how many things can be made. It's crazy when you think about it.