Some [TV-] shows have a small stable of directors, but also usually rely on outside directors.
Given the time constraints of broadcasting, a single show might have two or three episodes in pre-production, one or two episodes in principal photography, and a few more in various stages of post-production.
The task of directing is complex enough that a single director can usually not work on more than one episode or show at a time, hence the need for multiple directors.
Television vs Film Directing:
Film directors usually have most of the creative control, but in television this control tends to be more in the hands of the producer.
Although this is not always the case, the television director can be thought of more as someone who molds the show into the package requested by the producer, as opposed to someone who stamps their own feel on the production.
The Showrunner is usually the person responsible for a TV series.
TV's showrunners outrank directors:
For film directors, it's all about control.
Realizing it will be their vision that appears on the bigscreen, and knowing they'll be judged on whether the film is successful or not, helmers fight for final cut, casting choices and answer to no one -- except, possibly, the visiting studio suit or producer -- while ruling on the set.
With episodic television, however, directors walk a fine line between their own creative muse and the expectations of the showrunners.
From an interview with Jeannot Szwarc, who has directed episodes for several TV shows (Supernatural, Bones, Grey's Anatomy, Fringe, Smallville, Heroes,...):
Now being a director who travels from show to show can be difficult. Always fitting in. Always having to adapt to new styles. Do you have a philosophy that helps you?
Well, yes. The philosophy is that you are a guest wherever you go. You want to preserve your style and sensibilities, but you must also fit into the style of the show and into the group. ...
First thing is to try to make the show the best it can be within the parameters that are established. Try to improve the script as best you can. And to try to convince everyone to see it your way.