People tend to hire persons most like themselves ( i.e. people they like and get along with) rather than the person best-suited to the job.
This can be mitigated somewhat when hiring people with concrete job requirements -- a bricklayer, for instance.
But when you're hiring for job with ill-defined or poorly understood jobs (what makes a good manager?) then people punt and hire a "good person".
It's why management gets filled with "good old boys" and acquaintances and presidencies get filled with.. well you know.