...because Samuel L. Jackson is not a TV actor and just too expensive.
And yet he has publicly stated that he would like to play a part in the show somehow. Granted, SLJ as an ongoing regular is out of the question. But an appearance or two a year? Foe the really important episodes (Sweeps)? I could see it happening.
Yes of course and there seems also to be a trend with major movie actors to try out (well written) TV shows because it gives them more time to develop characters and an interesting acting challenge.
May not be the case here but it seems Jackson likes the Fury role and why not invest a week or two of shooting and have fun again (and make some money off it).