Early medieval evangelists had theological reasons for targeting their evangelistic efforts to kings. Kings were heads of political bodies; convert the head, convert the body.
In a summary of the Carolingian Renaissance, James Hunter (To Change the World, 59) offers a more pragmatic reason for the tactic: “the monasteries were outposts of evangelization. In this, the monks focused on the regional and local aristocracy rather than on the rusticus, the common people. As a rule, the higher up in the social hierarchy, the better for the church, for they were the ones who had the local influence necessary to diffuse the faith among their dependents.”
Rather than start at the bottom and put converts on a path of potential martyrdom, the monks started at the top and avoided the possibility of martyrdom. No doubt the monks agreed with Tertullian that martyrdom is the seed of the church, but they were hard-headed medieval pragmatists who didn’t court hostility when they could avoid it.