They believe this is in the nature of people to resist changes, especially when everything is ok. Things go well, so why bothering with changes. The truth is that those who simply do things as usually never achieve great goals. If a company gets complacent it may even die, overruled by concurrent who took on the challenges.
Similar rule works for quality assurance, defect prevention, and process improvements in the organization. I always face resistance when I speak of the need in code design, review, and unit testing. Even most persuading words may break down upon the blind "prove me why I should be doing it". This is very discouraging when people nodding heads in agreement on the meeting, who look as if to buy the idea, allow it silently die a month after.
This is not reasonable to believe that another preaching session in a while will change things for better. Repetition is good but it's not enough to foster right attitude toward the case. People who are responsible for using the change in the process must believe the changes are needed. And what is more important they need to believe that they need it personally to do their job better. If you fail convincing them in it, there is little change your ideas will get due support. Another necessary thing is management contribution. Management shall keep insisting on following best practices. They need to patiently explain what the purpose of a process is and how it is going to help the team to reach the goal.
Only buy-in from performers and consistent message from the management will do the job. If one of these is absent the initiative will die a silent death.