modified one designs

The boats that were lightened and improved down here were different from most of the boats in other countries, which can't be given the same treatment. The boats that received the lightweight treatment were designs like the Sharpie which were, even in their original heavy form, flat in rocker and section. Such boats are popular in Oz, where winds in the sailing season tend to be stronger than in most other places and there is very little inland/lake sailing. We also tended to add more crew weight to give them more leverage to handle our stronger winds. 

The Kiwis were similar in many ways but their boats tended to have more beam and less length; I suspect that this is at least partly because they had some nice protected waterways and therefore were more into day cruising (as was the case in the USA) and perhaps because they had less money per boat, smaller cars, etc.

This historical chance meant that Oz was in a unique situation when plywood arrived and allowed us to make lighter boats. If you have a skinny, flat boat with more crew weight than other places would give it, when you reduce hull weight you are left with a boat that is still sailing to its original lines, or something pretty close to it. Therefore the whole thing ends up working pretty damn well most of the time.

Records from the time the lightweights arrived also show that the real attraction was not the extra speed (although that was appreciated) but the economy and ease of construction, maintenance and launching/carrying across the many sandy beaches. The speed aspect only gained precedence in hindsight.

Boats in other countries tend to have more curve in rocker and section because they are designed for lighter average winds and for other cultural/technological reasons. If you take a whole lot of weight out of them, they end up with too much rocker and not enough waterline or waterline beam. They just don't work as well when you take weight out, and being lighter may also be of less use if you have light winds and different launching setups.

So while I'm a lightweight boat fan by preference and birth, there may be significant problems in taking what worked down here and applying it to other areas. It's not a case of one style being good and the other bad, just different solutions for different situations IMHO.