Those Who Can See blog has an excellent article about housing "red-lining" of in the 1900's. The article has some sections about the credit-worthiness of blacks.

A few selected quotes (though I recommend the entire excellent blog post)

Confirmation from a 1919 Chicago race commission report:

[...] A leading real estate dealer said that "when a Negro moves into a block the value of the properties on both sides of the street is depreciated all the way from $100,000 to $500,000 [$1,300,000 to $6,500,000 today], depending upon the value of the property in the block"; that it was a fact and that there was no escaping it.

...

The exclusive occupancy [in Chicago] of a block by Negroes is usually followed by less care of streets and alleys. [...] From the office manager of a South Side real estate firm: Much depreciation, he said, can be attributed to Negro tenants; they are much harder on houses than white tenants of the same station in life; they do not take proper care of the furnaces or plumbing, and the higher rents paid by them merely cover the cost of the additional repairs;... (10)

 White residents [in 2005 Chicago] were far more likely to report disorder than black or Latino residents living in the same neighborhood -- sensitivities that might explain, they theorized, why whites are relatively scarce in many city neighborhoods.  [...] As the percentage of African Americans in the neighborhood increased, the percentage of black [as well as white and Latino] residents who judged their neighborhood to be in disarray also rose.

Anyway, I'm going to end up quoting the entire article. And I haven't exactly addressed the question. The "Why"?

Because read the rest of CoonTown! Blacks just don't! Don't pay bills, don't control themselves, don't plan far ahead, don't have a well developed feeling of shame, ...