Quite often, landing one of those jobs comes down to knowing the right people at the right time. That is perhaps more true for media relations jobs than just about any others simply because you are talking about a profession that thrives on good communications and interpersonal relationships. People who hire media relations types tend to go with people they both like and have the skills the job requires.
That is another reason you see so many journalists landing those jobs. When it comes to relationship building, their are few jobs out there that better allow for that than journalism. Quite often, a reporter will make a good impression on someone in a company or an organization that is looking for a media relations manager and will be offered a job.