Yup I agree with Lucky Ladybug, it would cost them silly bucks to use the show VAs plus they would probably have to pay additional licensing for the rights to that
But then why did they do it for Fluttershy? That is the question.
Is it 100% the actual voice actress? I have that plush with voice box [did not even realise it was in there
] and while it sounds super close I could not say for certain that its Andrea Libman but thats more because the voice is not exactly clear.
Also these boxes are bolt ons launched after terms for releasing Pinkie and Rainbow Dash and Twilight were probably all agreed. Perhaps at the time BaB decided against the extra cost of carrying the voice boxes for these ponies but having seen it requested or the potential popularity of such an item when it came to Fluttershy they were happy to pay it. Going back and creating accurate voice boxes will add cost [but not necessarily high end profit] to 3 toys which have been out for months, have already been purchased by much of the keen market and are potentially going to be retired at some point. Unfortunately I bet it all comes down to them not being willing to $$ on a bolt on for an item which has already peaked in sales.