It could also be that the instruments are still on separate tracks, but that a 'deconstructed' intro like the one created during the mix for the original mono record just doesn't sound so good in stereo. Sometimes when you do that, it sounds OK in mono and you get away with it, but muting the tracks in stereo sounds too obviously like... you muted the tracks to create that intro. You can sometimes hear that you've muted a track more obviously in stereo. So perhaps they changed it for the stereo mix and left everything playing for that reason...?
And this seems to be the most "acceptable" reason to me if the change was indeed deliberate.
But if it wasn't intentional, it seems like neglect/an oversight in the remix process. And it seems like the faded out piano in the intro was possible to execute because that piano part *is* correctly muted when the cycle of the song repeats after the chorus/before the second verse on the new mix (well... actually the piano fades back in on the second beat of the first measure of the second verse in the mono mix but it starts on the first beat in the new stereo mix).
I realize I'm one person with their own subjective opinion on the internet and am no authority on the matter. I have the maturity of a ten year old when it comes to dealing with differing music mixing decisions—even when the piano came in on the stereo mix of the Smiley Smiley version of Wonderful irked me lol.
Aren't I glad that these new mixes exist? Yes!
[/pedantry]