Because they wanted to restore the original 1940 roadshow cut, but did not have audio (or at least, usable audio) for the deleted Deems Taylor introductions. They only had picture.
Really, they should have left those scenes out and presented the Corey Burton redubs in the bonus features, and an explanation why they’re redubbed and why they’re not in the movie. It’s not like kids give a crap about those extra scenes anyway, they were the most boring part when I was a kid and it took me years to appreciate them.
On some future release they should just include the shorter cut with seamless branching or on another disc, with Deems Taylor’s voice intact. It wouldn’t be that hard.