Most of the companies were "On the Fence" on building the 17.5's too, until ROAR legalized them and made a set of class rules for them.
I know ROAR was on the fence about them too, I spent some time emaling and pm'ing them with info we had found with almost 6 months of testing them in different types of vehicles.
The 17.5's were also built for us - but as with any product, the MORE uses you can find for it - the bigger sales will be - the easier it is to get someone to build. Which is why I'm really excited to see the 21.5's being used in the VTA. MORE Uses, More sales, More availability.