Because it's not so much about SHIELD as it is a Level 7 splinter group, acting independent of the primary body. You read SHIELD in the comics, and it's about Nick Fury,Maria Hill and their close compatriots. But that would be an even more expensive show, and you can't get Samuel Jackson for more than a fleeting cameo.
They might have to move up the reveal of what's really going on with Coulson, to goose ratings.