The nobles are the descendants of people who were early supporters of the Lord Ruler. The Lord Ruler gave them the gift of Allomancy because of their support. This has been passed down through the generations, but their Allomantic gifts have grown weaker over the years. At first there were few Mistings, but now Mistings far outnumber Mistborn. Allomancy is the main reason the interbreeding of nobles and skaa was made illegal by the Lord Ruler. The Lord Ruler had more control over the Nobles, so he didn’t want the skaa to have Allomancy.