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Because the index is not associated with it?
A primary or unique constraint either implicitly adopts a suitable existing index or creates its own.
A foreign key does neither, although it is generally beneficial that foreign keys are indexed for query performance and to avoid some locking scenarios.
Unique or Primary Key cannot enforce Uniqueness of a column with out a Index on that column. Hence Oracle will create an Index by default if an index does not exist on that column while createing these two constraints. So an index is always associated with these two constraints.
But a foreign key does not enforces uniqueness on a column. And an Index on a foreign key is not mandatory. So even if you create an index on a foreign key column that index is not associated with the foreign key.