What is the Big Deal With This Proof?

Now this is certainly a proof sheet that I wanted to get.  Of course this is another sheet that I will never own.

I guess this is a starting point as to why these cards are the toughest to find.  They are all on the same sheet.  Did something happen to this sheet so that it ended up in lower numbers?  They certainly had to show up in the same number in factory sets.  Does anyone know more about this?

In no way do I consider myself an expert on Kellogg's cards.  Please pass along any ideas.