What was the rationale behind the Clan Protectorate being centered on Marik? I'm sure there's a story to it, but I don't know what it is. I assume the extinction of the Marik Commonwealth and the inception of the Protectorate must go hand in hand?
Clan Spirit Cat conquered Mark in 3135 while looking for sanctuary for their people. Clan Sea Fox assisted Clan Spirit Cat by providing transport and additional ground forces in the final battle. Julietta of Marik, the eldest daughter of Captain-General Jessica Marik, got on-planet right before the last big battle of Marik was going on, and left an impression on the Spirit Cat leadership. After the assassination attempt on her life, the Spirit Cats loaned some of their doctors to Jessica to help heal her daughter, and then brought her back with them to Marik to monitor her recovery. Julietta eventually started to identify more the Spirit Cats and then helped them take advantage of Oriente being distracted during the 3138 war with Andurien to declare their own statelet. Then when the Free Worlds League was reborn, it joined as an equal member-state.
Basically, Julietta realized her mother was going with Nikol (Julietta's younger sister) as her heir and after her life-and-death experience and recovery, Julietta became much more determined, and stuck it to her mother by getting Clan Spirit Cat to expand its influence, rather than be consumed by the Oriente Protectorate. Basically a "you-scratch-my-back-I'll-scratch-yours".
TLDR: The reason it's centered on Marik is that is where the Spirit Cats and Sea Fox started from. Rikkard Nova Cat believed Marik would be the sanctuary he was seeking, and then went from there.