The problem is that there are different physiques (the classic "meso, ecto, endo as an example) and differing combinations of muscle fibers between fast/slow twitch. Some people will respond better to certain types of training based on that. So many trainers use a cookie cutter approach for what worked FOR THEM and apply it to everyone else.
Second, most people confuse "strength training" with "bodybuilding" and use an approach meant to cause hypertrophy (bigger muscles as the goal) instead of focusing on just getting stronger.
Third, there is a point of diminishing returns. A martial artist only has so much time in the day and the best way to improve at an activity is to do that activity. Balancing the practice of their art with conditioning and strength training is hard for many people who can't do it full time. This is where the issue becomes really blurred because pro athletes take PED's that change their physique composition into something most "natural" athletes will not be able to accomplish. It allows them to grow/repair muscle faster and also have faster recovery times so they can do more.
So, looking at many "pro trainers", they can do alot of things wrong that still work because they are either working with genetically gifted individuals that respond quickly to almost any training or chemically gifted athletes that will respond quickly to almost any training, or a combo of the two.
Add that into the mix of "martial arts training" and much of what is done is still around because of tradition. Look at boxing even, there is a big argument about "roadwork". Some "traditional" boxing trainers will say that 5-10 miles a day was good for Dempsey (or whoever) so it must be good for my fighter. Other trainers will say that they just didn't know any better due to lack of exercise science and have their athletes doing intervals that mimic timed rounds. If you are a great trainer and produce champions despite your less than optimal training, and the other guy is spot on with the science but can't produce a champion, who are you going to listen to?