Why is this not using the index?

mongodb index question

> db.collection.getIndexes();
[
        {
                "name" : "_id_",
                "ns" : "test.collection",
                "key" : {
                        "_id" : 1
                }
        },
        {
                "ns" : "test.collection",
                "key" : {
                        "generated_at" : 1,
                        "captured_at" : 1,
                        "priority" : -1,
                        "modified_at" : -1,
                        "weight" : -1
                },
                "name" : "generated_at_1_captured_at_1_priority_-1_modified_at_-1_weight_-1"
        }
]
> 

> db.collection.find({}).sort({'modified_at': -1, 'generated_at': 1, 'weight': -1, 'priority': -1, 'captured_at': 1 }).limit(1).explain();
{
        "cursor" : "BasicCursor",
        "nscanned" : 83,
        "nscannedObjects" : 83,
        "n" : 1,
        "scanAndOrder" : true,
        "millis" : 0,
        "indexBounds" : {

        }
}
>