Why did banks and lenders not voluntarily contribute positive credit data before?
Some lenders voluntarily contributed positive credit data; as of April 2017, Equifax reported that 24% of lenders were sharing data. However, this didn't include any of the major banks.
There were a few reasons for this. One was that the banks took a long time to prepare their systems to report data to credit reporting bureaus in public mode (some banks, such as NAB, had been sharing data privately).
Another reason was that the data the banks held was regarded as an asset and it allowed them to make lending decisions and curate product offerings to their own customers. If they shared this data their competitors would be able to do the same to their customers as well.