Index is not used when doing a query with PostgreSQL CTE

Given I use the following table :

CREATE TABLE operations (    asset_id varchar(255) NOT NULL,    event_id varchar(255) NULL,    updated_at timestamptz NOT NULL DEFAULT CURRENT_TIMESTAMP);

and the following Index :

CREATE INDEX idxON operations ( event_id);

When I run the following query

WITH ROWS AS (SELECT    *,    ROW_NUMBER() OVER (PARTITION BY asset_id ORDER BY updated_at DESC) AS row_numFROM    operations)SELECT    *FROM    ROWSWHERE    row_num = 1    AND event_id IN ('bb87d265b3c82098f7cd6913a4eb6df2', '4780d32075328c6f6f0ebe4e7fd48662')    AND operation != 'DELETE';

the index is not used :

Subquery Scan on rows  (cost=7159.77..7909.77 rows=1 width=598) (actual time=48.159..48.160 rows=0 loops=1)  Filter: (((rows.event_id)::text = ANY ('{bb87d265b3c82098f7cd6913a4eb6df3,4780d32075328c6f6f0ebe4e7fd48662}'::text[])) AND ((rows.operation)::text <> 'DELETE'::text) AND (rows.row_num = 1))  Rows Removed by Filter: 20000  ->  WindowAgg  (cost=7159.77..7559.77 rows=20000 width=598) (actual time=38.352..46.491 rows=20000 loops=1)        Run Condition: (row_number() OVER (?) <= 1)        ->  Sort  (cost=7159.77..7209.77 rows=20000 width=590) (actual time=38.343..39.519 rows=20000 loops=1)              Sort Key: operations.asset_id, operations.updated_at DESC              Sort Method: quicksort  Memory: 2911kB              ->  Seq Scan on operations  (cost=0.00..467.00 rows=20000 width=590) (actual time=0.012..2.993 rows=20000 loops=1)Planning Time: 0.144 msExecution Time: 48.436 ms

If I remove the row_num condition, then the index is used. e.g :

WITH ROWS AS (SELECT    *FROM    operations)SELECT    *FROM    ROWSWHERE    event_id IN ('bb87d265b3c82098f7cd6913a4eb6df3', '4780d32075328c6f6f0ebe4e7fd48662')    AND operation != 'DELETE';\

Can someone explain why the first query does not use the index ?