Why do you think are the reasons why only few seeds germinate​

The first reason is that Thats because the seeds will remain dormant until they have the resources they need for eg: some seeds are frozen in winter so that they wont germinate until they have enough resources in spring .

The germination of a seed can eventually be affected by temperature, humidity and oxygen. Generally, the seeds remain viable longer at low temperatures, a humidity range of 3% to 20% and an environment with low partial pressure of oxygen. If some of these factors are not met, the seed can age or eventually germinate.

Explanation:

Seeds are the main reproductive organ of the vast majority of higher terrestrial and aquatic plants. They play a key role in the renewal, persistence and dispersal of plant populations, the regeneration of forests and ecological succession. In nature the seed is a staple food source for many animals. Seed germination often takes place on the surface of the soil, so the balance between the gain of soil moisture and its loss by transpiration to the atmosphere determines when the seed becomes saturated with moisture and begins to germinate.

Gamete formation.

Explanation:

During meiosis, even if the cell is divided into four different cells, the DNA is only copied/replicated once. The formation of gamete segregates gene pairs among the new cells.