Allowing other sailmakers makes the class more one-design and allows folks of different abilities and weight be closer in speeds.
Stock sails are cookie-cutter designed.., one sail for all, fat, skinny, tall, short, crisp helmsman, sloppy helmsman.
However, a big person needs more power than a light person. With after-market sails a big person can have a fuller sail designed which has more power.
A lighter person would want a flatter sail as they do not need so much power.
A fuller sail is easier to sail and has a larger band before going into an error zone, while a flatter sail has a narrower band of good zone. So, the crisp helmsman would opt for a flatter sail, while the sloppier helmsman might want the fuller sail.

And this has worked to perfection with the Wave Class. In Our local series races we do about 120 races per year (about 60 last year in Key Largo, and about 60 in Ohio, and in both cases, women and younger, lighter kids compete evenly with bigger folks -- We have 4 of us over 200 lbs). No one dominates the racing.., and that is pure one-design at its best.

And the Nationals two years ago was won by Kathy Kulkoski. Other women were in the top 10. Zach Grant weighed about 100 lbs and won the Worlds in Provo in the Turks and Caicos. Last year's North Coast Championship was won by my Daughter-in-Law, Leah White, who is just over 100 lbs.
And, on the other hand, I have won the NAs 3 of the last 4 years at 230 lbs.
No matter the weight, in the Wave Class it is anyone's regatta.

In other words, you can have a sail that fits your size and style built for you at a reasonable price. One sail for all is not the way one-design has ever been, i.e., Tornado, Thistle, Interlake, Lightning, et al.

Ever since I started sailing Hobies in 1980, I was told by all the top sailors in the TheMightyHobie18 Class that the first thing you did with a new factory sail was to take it to a sailmaker and get it redone or resewn. And for the same reasons I have described above.

Rick