Because those actually MAKING 'open' arrangers and WS's have decided to concentrate on the engine, and forget abut the interior of the car. Kind of like putting a modern turbo-diesel on a Model T.

The day that anybody makes an open arranger, with computer based sample loading and current I/O capabilities, and THEN adds all the content and ease of use that the vast majority of arranger players want, the world will beat a path to their door (as long as they don't make it unaffordable). Until then, they will remain niche products for a tiny niche market.

We all know (or at least, some of us remember) that most arrangers are not built around off the shelf computer parts, motherboards, etc. So cheap, off the shelf memory is not going to be possible to put in most closed arrangers. They are built around RISC and custom designed chips, so it's likely the FLASH RAM is proprietary, too. But no doubt someone more technical will take a look at Yamaha's FLASH RAM and see if an off the shelf alternative would work... Me , I very much doubt it!