Possibly availability. There seem to be quite a few ‘standard’ kits that are not listed at the moment.

The Overlord of Music bridges are the only headless ones available cheap enough to supply with a kit, and they are a long way from perfect and really need work to them to make them perform adequately.

There will probably be trademark issues with the small Steinberger body shape, so you’d have to have a kit that had a different body shape, though it wouldn’t be hard to make one that could easily be cut down to size if you wanted to.

I can get a new Spirit GT-PRO in the UK from £299 shipped and that includes a gig bag. Given that the headless kits cost more than than a lot of otherwise similar kits due to the extra hardware costs, and the extra costs of shipping and duty and the finishes, it wouldn’t make economic sense for me not to buy a real Spirit, given that the hardware on the Spirit would be far superior to that which came with the kit.