Courts are very particular about the kinds of evidence they will allow for consideration. Judges have to ensure the science being presented is solid, the arguments match the facts and that courts don't become a he-said-she-said war of words - even if sometimes, it comes down to which side you believe more.
One type of evidence courts seek to avoid is hearsay. This is information received from other people that can't be adequately substantiated. What surprises many people in car accident litigation is that many times, police reports - or at least significant portions of them - are considered hearsay. In many cases, even when the reporting officer is called to the stand, he or she can only attest to the information he or she personally observed - not what other people told them at the scene.
Because information-gathering from third parties is a lot of what officers do at the scene of a crash, that can mean big chunks of the crash report get excluded from evidence - both in criminal and civil cases.