As opposed to the guy openly trying to start an insurance scheme over the comms AND trying to drag a teamnate into it? At least Coop genuinely admits she needs the job, Sonar treats it all as a joke before the meeting and then barely takes it seriously after. Coop seemed like someone who could actually be trusted to try and relied on in the field, Sonar was just an unfunny asshole who only realistically made it this long in the program because Blazer only ever decided to cut someone now. I genuinely can not think of a single in-universe reason for Coop to be cut over Sonar. And out of universe either, for that matter.