licensing issue because todd phillips and joaquin didn't want to promote and sell merch to children or teens. there was a whole talk about merch between todd Phillips and an exec (i think) in an interview or BTS and how there are joker boxers/underwear being sold at retail stores and they did not want to give the impression that a character like this shouldn't be so accessible to the masses, especially kids and teens.
also joaquin isn't the type to like being in the lime light so there's a potential reason there on why the merch is so scarce. setting a price bracket means it would mainly cater to adults with disposable income instead of kids. the minimum price requirement almost acts as a deterrent for young people because who tf in their right mind would spend that much for a character like this.