Hydraulic pressure

Like anything retail, someone has to order it in, get the correct bits, put it together as a "correct kit", test to make sure it fits particular models, put it in their inventory and computer system, make a space on the shelf to store it, pay for and carry the stock until it's sold, offer warranty for the product, pack it up and send it off, sit on the end of the phone and answer questions, pay for warehouse space, insurance, power etc., etc.
So you either spend the time and buy bits and pieces from different suppliers with multiple delivery charges, or you spend time and fuel and drive around to different shops, and hope you order the correct bits, or you don't waste any time and buy from someone who has done all the leg work already and made it easy for you.
I don't know the actual kit, but that's the process for any kit from any good supplier.

I built a kit for my BX23S, since the new model uses flat faced connectors. I can say that it wasn't all that expensive, but the research to find what I needed, then the time to order it, wait for it to come in and then assemble it.... I wish I could have just gotten a kit. I guess it all depends on how much your time matters to you.