why would Oracle require hint to use bitmap index? does not pick up unless hinted

we have oracle 8i enterprise version, we created bitmap index on field. cardinality is 10 disitnct values over 90, 000,000 rows. we anlyzed both table and index. we run qurey that looks for value that exists in only 1000 of the 90 million rows, the explain plan is full table scan (bad). we pick value that exists in 70 million of rows an dit also does full talbe scan(ok).

we throw index combine hint and it picks it up. both not without hint. Isn't oracl esupposed to pick it up automaitcally based on optimizer rules?

here is index def:

CREATE BITMAP INDEX X_BT_SALES
 ON TF_SALES(SALES_DEPT_TARIFF)
LOGGING TABLESPACE DATA01 PARALLEL 3 status is valid

select * from TF_SALES ss where ss.SALES_DEPT_TARIFF = 'MULTI TIME OF DAY' Received on Fri Dec 08 2000 - 14:21:33 CST