There are several explanations provided by the Sages with regard to why one person was created alone, and from him all of humanity developed.

The reason why Adam was created alone is to teach you that with regard to anyone who destroys even one soul from the Jewish people, the verse ascribes blame to him as though he destroyed an entire world. And with regard to anyone who sustains even one soul from the Jewish people, the verse ascribes credit to him as though he sustained an entire world. Another reason is due to maintaining peace among people, so that one person will not say to another: My ancestor is greater than your ancestor. Another reason is so that the heretics will not say: There are many authorities in Heaven. Another reason is to tell of the greatness of the Holy One, blessed be He, as when a person stamps several coins with one seal, they are all similar to each other. But the supreme King of kings, the Holy One, blessed be He, stamped all people with the seal of Adam the first man, and not one of them is similar to another. Therefore, each and every person must say: The world was created for me. (Mishna Sanhedrin 4:5)