Oracle SQL: Insert select

Can someone understand following schema: Why two first two will fail? In the will succeed case, why one could select from one column zip_code?

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CREATE TABLE zip_new (
zip_code     VARCHAR2(5) NOT NULL,
state_abbrev VARCHAR2(2) NOT NULL,
city_name    VARCHAR2(30));

-- will fail
INSERT INTO zip_new
SELECT *
FROM zip_code;

-- will fail for a different reason
INSERT INTO zip_new
SELECT zip_code, state_abbrev, city_name
FROM zip_code;

-- will succeed
INSERT INTO zip_new
SELECT zip_code, state_abbrev, city_name
FROM zip_code
WHERE zip_code IS NOT NULL
AND state_abbrev IS NOT NULL;

SELECT * FROM zip_new;