Your question about why any individual physician chooses to practice a specific way is not answerable, but the underlying question about how physicians use evidence to make decisions is what I will address. This is from a perspective in the USA, but I believe that much of this applies globally. (But please feel free to add answers from other countries, as I'd love to hear how things work elsewhere!)
Guidelines (see also hierarchy of evidence here or here or here) are generally not rigid rules, they are consensus best-practice recommendations from a professional organization, based on evidence as it emerges. Guidelines are excellent tools for decision-making because they provide a consensus of what many experts in the field have assessed that the current evidence from all available studies best supports.
But Guidelines take a while to form and a while to revise or overturn, so they aren't necessarily reflecting cutting edge research. Additionally, there are sometimes even conflicting guidelines from different organizations on a topic - for example frequency of mammogram (yearly vs every 2 years) is currently a hot debate between the major organizations that address women's health, with valid viewpoints on all sides (balancing frequency of rapidly progressing cases vs the harms of false positive rates and complications of unnecessary biopsies etc etc, which is a huge discussion on its own).
There is no single organization that issues guidelines for all physicians. There are professional organizations for each specialty and even subspecialty (like AAFP ACOG AAP ACP etc), professional organizations for major disease processes (like American Cancer Society), national organizations that look at how guidelines apply at a population level (USPSTF, NIH), and independent organizations that provide EBM (evidence based medicine) tools for a fee that try to unite all recommendations and provide a discussion of how to approach decision making (UpToDate, Dynamed, and to a degree Medscape or Epocrates, etc).
When a new guideline comes out that creates a paradigm shift, how does a doctor decide whether to follow it for an individual patient in his/her practice? When guidelines contradict, which does he/she choose to use? When a new study comes out that challenges current guidelines, does he/she change his/her practice based on the study, or wait until the professional organizations have digested it and weighed it against the current evidence, and issued new guidelines?
There is no single answer to those questions. Physicians are trained to interpret and evaluate the strength and applicability of evidence, from guidelines to individual journal articles. If there is a standard of practice, and the physician chooses to deviate, there very well may be good justification for that.
For an over-simplified example, if a physician who has been practicing a long time has had success in treating patients with the old paradigm, and the new paradigm is controversial, or the new paradigm was only non-inferior without clear mortality or morbidity benefit, they might choose to stick with what they have seen work. Medicine is called an art for a reason.
This is an interesting article that might help expand understanding of physician decision-making: https://www.elsevier.com/connect/why-arent-all-physicians-using-clinical-practice-guidelines.
This article shows how the AAFP processed and recommended approaching the new ACC/AHA guidelines, to give another view into the complexity: https://www.aafp.org/afp/2018/0315/p372.html
So, what about LDL guidelines? It's a little bit controversial. I would say most newer physicians are going by the new guidelines, as that is what they learned while training. But a discussion about how to decide whether or not to follow those new guideline is not a short, concise answer. If I find a good article on it I will add it here.