Svarog, many reasons… the main because he is the god of Sun, and we know how important that fact was to the old Slavs. All the things were "scaled" by the movings of the Sun, suns changes etc. He had an very important and crucial role in the lifes of our grandfathers.
And, like tomo said – that is the reason he was the Father of all living things. The Father!

Interesting, I wondered why our ancestors were so focused on about the Sun. I know it was to help with agricultural reasons but was there anything else? Now, I need to read more about Svarog..