Searchable Custom Field doesn't appear in Quicksearch options list and is not a sort option in reports
The quicksearch function is for searching contacts, and therefore doesn't call to any of the other components of Civi for data fields. 

As per Civi's Docs, if you're looking for data regarding a specific component (i.e Memberships), each component has it's own search function. In Find Memberships, this should also show your custom fields to allow you to search from.