Regrettably, the agency does not have to explain anything to you unless you are a member of a protected class and file a discrimination action against them showing a prima facie case. Neither dose the agency need to review any background checks, with one exception — CORI, if you authorized the agency to obtain it.
If you are in a protected class, i.e., age, gender, sexual orientation, etc., and can find some evidence of discrimination, I urge you to contact the Massachusetts Commission Against Discrimination (MCAD) who prosecute the case at no cost.
If you think it is more personal, I would review your references, social media postings, and anyone who may have been interviewed during a background check.
This is not legal advice and is not intended to create an attorney-client relationship. You should speak to an attorney for further information.
I wouldn't be so suspicious. As you have stated, there is no reason that they would not want to find work for you. If they place you, they get paid, therefore they have a strong incentive to consider you for any and every job for which you are likely to be considered a strong candidate. Unfortunately, the economy is still slow, and staffing agencies take a hard hit during times like these. The agency probably does not have nearly enough positions to go around.
Still, there is no harm in contacting the agency again and inquiring about positions that may be available.
This answer is provided for guidance only. DO NOT rely on it as legal advice. We DO NOT have an attorney-client relationship. You should contact an attorney in your area for a one-on-one consultation before pursuing any action or making any decisions.