While the Taipan 4.7 has gained popularity in the US the 5.7 never did. There were only 3 5.7's imported to the US and only about 35 total built worldwide I believe.
In my opinion there are two kinds of catsailors - the racers and the pleasure sailors. The racers are the ones willing to spend the big bucks to have the latest, most competative boat they can afford that is being sailed one-design or Formula. Pleasure sailors want to spend as little as possible, so as to not cut into the beer budget
Thus the Taipan 5.7 is a boat without a country - the racer's don't want it because they can't race one design with it and it's way to expensive for what most pleasure sailors are willing to spend for their hobby. The 4.9 on the other hand has enough boats here in the US to race class in some areas and F16 in others. It can also be sailed solo or with crew, thus justifiying it's higher price.
The Taipan 5.7 can only be sailed Portsmouth here, it's 80 lbs too light to be an F18.