In my opinion, despite JKR's claims that she tends to read outside the fantasy genre and rarely interact with anything related to it, the little beaded bag screams Bag of Holding to me
(from D&D: Bag of holding - Wikipedia, the free encyclopedia). The lack of availability in that case is because they are rare and expensive items, meaning the magic creating them is high-level and complex.
One could assume the same for Hermione? After all, she's meant to be advanced beyond her years when it comes to magic.

Seems odd after all that trunk-hauling, but as prestigious as Hogwarts may be, the money required to give every student the equivalent of a Bag of Holding wouldn't realistically be available to them.


And as for the epilogue, I got the impression that the point that was being made was that everything in the lives of the characters had improved and there hadn't been any post-Voldemort troubles since he had been defeated 19 years ago. JKR was not giving us extensive details on the post-hogwarts lives of all her characters, but showing that all was well, giving us a little snapshot example of that. Cheesy as it was, her point was made with the very last line. "All was well."

Or it could just leave it all open for a whole new series of books ... just in case JKR gets bored some day a decade from now.