Analog pass-thru and SACD/DVD-A mystery

From what I understand SACD/DVD-A players will require the receiver/pre-processor support analog pass thru. They, its claimed, won't work with the digital inputs accepted by the latter systems today (and fed by the CD and DVD players).

Why is this so? What is blocking the SACD/DVD-A players from outputing digital signals (like their CD/DVD ancestors) so they can plug into existing pre-processors?

Can we expect, say over 2-3-5 years, to see SACD/DVD-A players output digital signals?
1. What's stopping them is that the S/PDIF input on the "latter systems today" is inadequate for high-bit-rate, multichannel signals.

2. A Fire-Wire interface system is forthcoming. Sony has just released a player and receiver and other have similar which will be adaptable. I expect there will be several at CES in January.
Let's not forget the copy protection issue. The reason why FireWire is an acceptable solution to this problem is because of its encryption, which would render attempts at copying the music useless.
Actually, the encryption could be implemented in any hardware mechanism. For example, Meridian splits the 6 channels of DVD-A into 3 encrypted S/PDIF connections. Clearly, copy-protection is accomplished but it took 3 digital links to accomodate the bandwidth.
Not exactly true; the DVD Forum has approved only certain physical carriers for encrypted digital interfaces, and those are spelled out in Forum documents as 1394 and (think it was..) USB. SPDIF as a legacy medium was never allowed; however Meridian has gotten around the issue in a proprietary fashion of their own. The DVD Forum permits proprietary digital interfaces if they pass muster with the forum.
Approval is yet another issue. To make the discussion broader, there are proprietary SACD interfaces, such as the Accuphase.

In short time, we will see approved, standardized and encrypted digital connections for both.