You are making the same observations as many designers..
There are of course many reasons, but I would think that the yacht brokers are still playing a dominating role in the selection of designers. Read what one of the major brokers say to prospective new-building clients on their website;
"Getting a select few designers to work up initial proposals allows you to develop your brief with fresh ideas, while deciding which you feel most comfortable working with. Simultaneously a XXX broker will be short-listing builders, suitable for your project ..... .... all taken into account, as are resale value and weighing up benefits of a strong ‘brand’ builder and designer versus gambling on the next ‘name’.... ... Even if your heart is set on a particular designer and builder before you talk with us, it might just ease any concerns"
To become a "next name" has less to do with design qualifications than with the ability to sell yourself. This is of course a fact not only for designers.
However, I think that today the buyers of new yachts are becoming more used to the internet when gathering information and they also know about WYSIWYG, What You See Is What You Get. In the past a naval architect was the king over the entire construction and designers were not heard of. This changed with Jon Bannenberg above all, talking of yacht design as an artform and without any previous knowledge of yacht construction he showed that a designer could change the industry.
Since then a new breed of designers have emerged, some are still better as interior designers but have built their "brand names" through the brokers and are designing the exterior as well. If you should ask me to name some good designers, I could only give you a handful that I know of, but as you say this is not a politically wise comment so I stop here...