Here's multiple reasons why the sarutobi forehead protector should be a leaf jonin standby item:
1.In the anime it is worn by a leafie so it would make no sense seeing sandies or misties rocking this protector.
2.It already has jonin requirement which makes no sense(to me at least) since right now it is an event item.
3.It can't be traded just like the items from the chunin shop. 95% of the event shop items are tradable.
The amount of cosmetics that leaf's jonin standby offers is way lower than the amount of cosmetics sand's jonin standby offer.