Answer:
Because Sun has lower metallicity
Step-by-step explanation:
Metallicity means the presence of elements in any objects heavier than hydrogen or helium in abundance.
Though the Milky way galaxy is of about 13 billions years, Sun is only 5 billion years old because it took time to collect dust and gas for the formation of such big celestial body. Its metallicity is low which means it has only lower mass elements which took time to combine in order to form heavier elements thus producing light and heat.