Because the football and non-football sides are inextricably linked.
Having someone else run the football operations is unquestionably a good thing. And, if run well enough, it may in itself assuage fan concerns. If United are successful from a football POV then most of the heat leaves the ownership debate.
However, the use of revenue generated by the club, investment into the club and the management of debt remain issues that directly impact football operations. Without full control of the club, INEOS will to some degree be operating with their hands tied in terms of football operations. And the overall direction of the club remains under Glazer rather than INEOS control.