User blog comment:B214/Talk About What You Want! 2/@comment-74.77.52.225-20140801025045/@comment-1359322-20140801094229

Forming a peerage isn't just going around gathering members and it's done. Issei isn't in a situation like Rias where she can just use the Gremory's asset for her own use. In Issei's case, he has many things to do when he becomes a King. Issei has to consider factors such as the funds, the territories and how to take care of his servants. All this could easily take tenths to hundreds of years to set up.