Basically what inhibits a person from Atlanta is not their legal or technical disadvantages, but those perceptual ones - large parts of society, and often police in particular, holding prior assumptions of criminal disposition and unwillingness to better themselves.
I'm Irish and we have very few Roma people but we have a similar and related ethnosocial subgroup. Imagine how difficult it would be for a 'gypsy' person to attain significant political power in our home countries, and how even if elected/appointed to a position of responsibility everything they did would be undermined by attacks on their heritage, regardless of their legal or intellectual eligibility for the role.
I noticed very soon after arrival in the US that I was assumed to have much greater potential than members of disfavored ethnosocial minorities. I say ethnosocial because I'm thinking of a former roommate from Mauritius who felt that his French accent and foreign origin gave him substantial social advantages vs American blacks whose negative qualifiers were already assumed.
Overall the Roma population probably has it worst. Followed closely by the immigrants.
I'm not a good judge of Why because I don't have good insight. The Roma seem to have your usual problems of being below the poverty line, a culture that heavily discourages improving your lot in life, and a strong culture of expecting govenment handouts. If you asked a Roma I'm sure their explanation would be different, this is what it looks like from the outside.
The immigrants, well, they're immigrants. New culture new life new jobs low prospects due to social standing and so on. Immigrants always seem to struggle.