It's something they only did for a few editions, and then I think they stopped a couple of years ago. I don't recall a reason given.

Oh, it was not "just a few"
editions. Definitely not.

There's never a proper explanation, but I can guess at least one..

As far as I'm aware, that has always been the case. You should always get a new card that you didn't have, (of the lesser values I believe, so silver or bronze, not gold and diamond.) It has been this way for as long as I can remember, it was at least this way the last card event. I just opened about 5 packs myself and got at least 1 new card in each pack,

I believe it only takes into account the cards you have activated, so if you are just holding onto cards and not activating yet it does not count those.

That was what I was thinking.
Wasn't holding any cards except jokers, so that isn't the issue.

I wonder what's up with that.. certainly will be tracking the cards I get in packs - provided I decide to buy more.