What was the solar nebula like? why did the sun form at its center?

The Solar Nebula consisted of spinning dust with the sun at the center.

Mass concentration at the center lead to the Sun to form at its center.

Explanation:

The Giant Molecular Clouds that formed the Solar Nebula consisted of matter in the form of dust particles which consists of 9 percent helium and 90 percent hydrogen and 1 percent other elements. When the Giant molecular clouds collapsed and the mass of dust started rotating most of the mass concentrated in the center forming the Sun at its center. The rotation also caused the flattening of the solar system. The Solar Nebula consisted of spinning dust with the sun at the center.

The planets formed by the leftover mass of the Solar Nebula.