Why did Agnew command Force K?

Why was William Agnew selected to lead Force K despite the apparent seniority of another cruiser captain?

Cunningham trusted Agnew's record and aggressive operational style, making him the preferred leader for the raiding force.

Cunningham's choice rested on trusted operational temperament rather than simple seniority. Agnew had commanded varied ships, had served with Cunningham before, and is presented as an aggressive officer suited to pushing a Malta raiding force hard. The comparison is with a more conventional peacetime-cruiser command profile. Fit for this particular mission outweighed the apparent seniority of another cruiser captain.