Hollywood may do the white savior/white co-lead thing for financial reasons, but try asking yourself
why the demographic demands it in the first place (or why non-white characters are occasionally whitewashed for this demographic?) and why they cave in to such demands rather than change the trend with all the storytelling power they hold. We can't do a movie with two black leads. Why? Money. What of it? They won't pay to see it. Why? ...
It goes back directly to racism. The demographic only chooses to accept prominently featured blacks in roles in which they either need to be rescued, reformed, educated, or assisted in some way by whites to kick things off and finally accomplish anything (see Django, The Help, plus other examples below).
http://www.complex.com/pop-culture/2011/09/the-10-lamest-white-savior-movies/
In this very thread, we have people throwing excuses left and right how a black character in an otherwise completely fictional, ridiculous over-the-top story (1 vs. 20 shootout!) could pull off rescuing himself and exacting his revenge without the help of a prominently featured white character. It has nothing to do with Tarantino being unable to craft a Schultz-less (or reduced role) story, and
everything to do with audience expectations that a white actor be featured prominently somewhere/anywhere on screen as co-hero.