Different weights for different manufacturers hulls are due to several reasons. The Hobie company (as the main and prime example) has, from the time of their first cat, never, apparently, considered "weight" as a major consideration when it came to marketing and manufacturing their cats. They have always relied on their "advertising and promotion" (which has worked well). It has only been when they have faced stiff competition from other cats based on weight-orientated performance that they have reluctantly reduced their collective hull weights. It’s not that they couldn’t reduce their weights at any time and still retain “strength”, but more so that to reduce weight in a hull and still retain strength requires more time and consequently, time being the biggest variable expense in manufacturing, more cost, and unless the price goes up to reflect those extra cost, there is then less “profit”. So weight, or lack of it, comes down, primarily, to a “cost effective” exercise, regardless of the types of materials used.
To one degree or another, this same “ethic” has governed all the “major” manufacturers of the most abundant classes of cats.
There is still of course that “big” question as to why the F18’s minimum weight is so “high”? The common answer is that it keeps costs down as it eliminates the need for the use of, so called “exotics”. This of course is a “furphy” as the weight of any F18 cat could be made at a greatly reduced weight than it is at present without using any “exotic” materials or sacrificing any strength.
My personal suspicion is that “Hobie” had an overly influential hand in the weight “rules” in the class’s early conception and therefore the final weight was retained at Hobies “comfortable” weight of the old TheMightyHobie18 (just my own suspicions)