Here's the problem: If there isn't enough evidence to convict, then putting it in front of a grand jury might end up with a trial and acquittal, which doesn't serve Obama's reelection, although the race hustlers will get lots of air time and riots out of it, but that also doesn't benefit Obama's reelection. OTOH, if they decline to prosecute, then the meme of a white power establishment that's covering up the crime gains traction, which also benefits the race hustlers, but doesn't necessarily create a flashpoint for nationwide riots. Obama needs this to simmer, but not boil over.
And, don't discount the Latino vote. This could really backfire on Obama. Black/Latino relations in inner cities are not good, despite what liberals would have us believe. The FBI crime stats lump Latinos in under the white category, which drives up the white numbers, but black/white homicides tend to be black/Latino homicides, and the decision to treat Latinos as whites for the purpose of ginning up hate crimes hysteria won't go down well in the Barrio.