It is just an entirely different product.

Digital pianos are made for 2 markets, those for trained piano players who just want a replacement for an acoustic piano with no tuning needed etc, and generally get a few extra harpsichord or string voices thrown in, and those that want a piano action but also with rhythms and more voices thrown in.

These are only similar to keyboards in the fact that they have rhythms and voices. The cabinet, speakers, weighted and graded keyboard and large piano samples are designed for piano playback first and foremost, the rest is extras.

The keyboard is designed as an easy play workstation, period. The 65 was the first product with a piano sample equivalent to the high end pianos.

The disks and formats etc were never designed for compatability or ever promised to be compatible - basically different products for different markets.