Why "should" they? It may be obvious to us but Meredith was not our daughter or sister. We can look at the case more objectively. Place yourself in their shoes. They were told from the beginning by the police, the prosecutor and their lawyer that AK and RS killed MK. The evidence, at the time, seemed to strongly support that. The British media and public opinion supported that. The first trial concluded guilt. The third trial concluded guilt. Other courts ruled that multiple people were involved (however flimsy was their reasoning). Families have a very difficult time believing that defendants were wrongly convicted. There psychological/emotional reasons for this. It's not usual. I doubt the Kerchers will ever believe AK and RS are innocent.
I believe this early misinformation has shaped their views up to the present. I doubt that their lawyer, Maresca, explained to the Kerchers that there was no real, credible evidence in the case against Knox and Sollecito. To do so would have been against his interest in maintaining their civil case and potentially winning a financial judgment.