The mystery of Jean Alesi's Williams F1 contract

Jean Alesi signed a deal to drive for Williams in 1991, which would have placed him at the team for one of its most dominant periods. But he never drove for the team, and had to struggle with poor Ferrari machinery as Nigel Mansell and Alain Prost won championships for Williams instead. To this day Alesi doesn't know the full reasons behind Williams baulking at making the deal public at the time, prompting several theories (involving Mansell and even Ayrton Senna) that could be behind his missed opportunity. Why do you think the deal never became reality? And what could Alesi have achieved if he'd driven an Adrian Newey Williams in the 1990s?