> Specifically why didn’t those “conferences and other means” stop the systematic corruption that led to the papacy selling church offices and abusing indulgences?

The council of Trent did reform indulgences. So the council did succeed. The steps took then to improve the education of church officials also had a positive impact.

The matter of offices is quite a complicated subject, as it had long been the tradition in the church for kings to appoint (or at least nominate) bishops and other church officials withing their kingdoms. The investiture controversies were about the parameters about kings doing this. And the educated class moved from the church back and forth to royal service, or did both at the same time (e.g. Cardinal Richelieu), with appointments in the church having the same politics about them as appointments in any royal court. It is difficult to say when the influence of monarchs over church appointments becomes corruption, owing to the complexity of its history.