Mitt Romney has two and exactly two qualifications to be President. He was a successful businessman, and he was governor of Massachusetts. As he spent much of his time in Massachusetts passing an individual health insurance mandate, raising fees for government services and advocating for carbon emission limits, so he can't really run on his political record. He's left, then, with running on his record as a businessman.
He wants to be given tremendous control over ~20-25% of the economy, but he doesn't want to give voters the evidence needed to support his business acumen. By refusing to divulge details of his activities at Bain, or to release his tax returns, he strips his claim to the Presidency down to being a rich guy. He doesn't tell us how he got rich or how he can use that knowledge to help the economy. In his previous term in public office, he led his state to a 47th place finish in job creation, so it's not like his skills at transitioning private skills to public service are evident for all to see. Without releasing his Bain and tax records, what real evidence, besides his nebulously-defined net worth, supports his candidacy?