That is easy to answer.
1) Everybody learns differently - some learn best though sound that they’re hearing.
2) Repetition. After reading to a youth for so many times, it could be useful to let the audio do the heavy lifting just by using replay value.
2a) In regard to repetition: it shouldn’t be too difficult to have/use as a tool to replay select minutes 17-23, pause, then play 17-23 again, as many times as is needed. For example. Or whatever segments are necessary/useful in that moment.
There probably are other reasons, but I don’t question that really. I think the whole series should be available and ready for use even if only one single person wants it. That one person enough deserves the choice.
Isn’t the purpose of McGuffey’s Reader to teach reading? How are you going to learn to read if you don’t see the text.
I would assume this is in the public domain and freely available to anyone on Internet Archive.