Same reason you can’t buy anything but crap hotdogs in Central Park - vendors pay millions for the exclusive contract to sell and then they sell crap at high prices to cover their investment.
No competition, no innovation, no reason to not gouge tbe punters.
It's not just a Central Park problem: a lot of north america basically forbids the sale of anything from a cart beyond pre-cooked hotdogs and burgers (putting it on the grill is mainly for show and to warm it up).
Kinda makes sense because of lack of hand-washing facilities, but those aren't impossible to set up.
No competition, no innovation, no reason to not gouge tbe punters.