Bad decisions get made. A lot of times, it's because whoever was paying for the new remaster would not authorize spending enough time and money to do it right. It basically boils down to, "we need this done in 6 hours." It may be possible that the stereo re-recorded "soundtrack" album wouldn't sync up to the animation, so that might be one reason why it wasn't used. I've seen issues like that before as well. There are techniques to create what my old pal Rick Chace used to call "Fat Mono" which are not too disturbing -- at least, if they don't add any phase issues or reverb to the signal.