Why did fusion stop abruptly in the early universe, preventing the initial creation of elements like carbon or oxygen?

Answer

Rapid cooling closed the window for further fusion.

The universe expanded and cooled rapidly after the initial formation of light nuclei, causing the critical window for further fusion steps (to create heavier elements) to close prematurely.

Why did fusion stop abruptly in the early universe, preventing the initial creation of elements like carbon or oxygen?
ElementastronomyuniversehydrogenstarscosmologyElements