Cause from what I read, "The sound editing and re-recording team began by preparing a four-track master mix (Left-Center-Right-Surround), which would serve as the basis for both the 35mm and 70mm stereo versions. The master mix was dubbed into a matrix-encoded two-track Lt-Rt (Left total-Right total) printmaster for use on the 35mm Dolby Stereo prints. The same four-track master, with some enhancements, was also used to create the six-track version. In comparison to the 35mm Dolby Stereo version, the six-track soundtrack during playback offered discrete channels, greater clarity and dynamic range characteristics, and special low-frequency content (bass extension, or "baby boom")."
So basically, how I understand it is that instead of getting 4 discrete channels(like we would with the 6-track 70mm soundtrack), we are only getting two channels of discrete sound with the 2.0 DVD soundtrack...which won't sound nearly as good.