Medical guidelines are put in place so there are fewer questions about what are correct tests and treatments for patients. They can be anything from - do you need stitches for that cut or do you need an EKG? The criteria used can be anything from age, general health, behaviors, diet, and much more.
Behavior is a big one. This is drinking, smoking, eating patterns, exercise, etc. But if behaviors of the general population change, the guidelines need to be modified to adapt.
A generation ago, more Americans smoked. Guidelines were developed for low dose CT scans for current and former smokers - designated as those who had quit in the last 15 years. They were shown to greatly reduce deaths from lung cancer as it was caught early for many patients. The guidelines were used to determine who would receive this annual screening and for whom doctors would recommend the scan and the insurance companies would pay.
Now as fewer Americans smoke and do not return to smoking, fewer are eligible for the low dose scans and have their insurance companies cover them. There is a concern that more cases of lung cancer will not be caught as early resulting in more deaths.