Maybe because it's used elsewhere in the standard library? SinglyLinkedList is used in the standard library in std.Thread.Pool, and std.heap.ArenaAllocator. Do

Maybe because it's used elsewhere in the standard library? SinglyLinkedList is used in the standard library in std.Thread.Pool, and std.heap.ArenaAllocator. DoublyLinkedList is used in the http client's connection pool, as well as the ObjectCache for the package manager's git client.