Why do some seeds germinate only in the dark?

It may be easier to answer the question "Why are some seeds inhibited from germination when in the light".

The key to this phenomenon is Phytochrome and you need to research that pigment and its different forms to write a full explanation. You will also need to find out about RED and FAR-RED light.

Phytochrome is present in one form in white/sun light and present in a different form after a period in the dark. Normally, the light form decays to the dark form after a few HOURS, but it is converted back again after a few MINUTES in the light. So, a seed on the soil surface gets enough light to keep the dark form of phytochrome low for sufficient time to prevent germination initiation. It is only when the seed is in permanent darkness that the dark form of phytochrome is active for long enough to trigger germination.

Obviously, this prevents such seeds germinating until they are buried.

This mechanism is even more interesting ... some seeds will ONLY germinate in the light (I know these are NOT the seeds you asked about), but if the light has first passed through leaves, then germination is inhibited. This enables such seeds to germinate as soon as they receive direct sunlight, but to remain dormant whilst they are under other plants which would give a developing seeding too much competition. It turns out that the spectral quality of light that has passed through leaves is such that it converts the phytochrome to the 'dark' form (as I have called it) - even though the seed is on the surface of the soil.

I hope you will find this a fascinating area to research.