Hard to say why they do that, perhaps there are just more kits being sold than Robart can keep up with?
I hope it doesn't work the way it did when I tried to get them. That was in the first part of July and they said they'd be in stock in mid-July, then it was late July, then early August, mid-August, late August, early September, mid-September, I placed them on back order, late September, early October, mid October, they finally notified me in late October that they were in stock but it would be several days before they knew if I would get them. They finally shipped them in late October...
I was on Tower's site last night looking for some other stuff and wondering about the gear, specifically if the tailwheel gear was in stock. That's when I noticed the mains were back ordered again...