If Sessions appoints a special counsel then they would have lots of independent freedom. Sessions can keep an appointed prosecutor on a short release and even replace them without much backlash. It's better this way.
Buber is just the initial prosecutor appointed by Sessions, such a huge conspiracy will require many prosecutors and many hand juries so this is just the beginning. The fact that there already is a grand jury and prosecutor should be a sign of good things coming.
Huber