on a flip side what is the point of releasing it now vs before, seems some malice intent on part of BBC. Indian Supreme Court already aquitted Modi of the Riots in 2012.
But to answer your question, the documentary and its timing is the result of a UK report created by the UK consulate after the riots was made public within the last year or so because UK law requires such documents to be made public after a certain amount of time.
Finally, Modi being acquitted in an Indian court of law doesn’t mean innocence. Even ignoring any questions of corruption, etc, India does not have one of those legal systems which decides whether a defendant is guilty or innocent. India’s court system only decides whether someone can be proven guilty.
An acquittal simply means that one cannot be proven guilty, not that they are innocent. The not guilty verdict can also be the result of a lack of evidence (which was the case in this situation), or due to procedural mistakes, etc. which may lead to an acquittal even if someone is guilty.