If they don't nail down the cause precisely, they will have to fix any long shot possibility that anyone can demonstrate might have caused the accident.
Machinbird point above reminds me of somethinng I have pondered in the past:
Often when reading failure reports the root (or major contributing) technical defect is shown to be something that would have been detected had the same people reviewed the mission in advance.
This has led me to wonder what would happen if the failure board had convened before the mission was launched.
A number of plausible failures have been identified in this case, rather proving that one was the cause before fixing it why not fix all of them, at least those that carefull analysis show unlikely to have unintended negative consequences.