Well from what I hear, it is due to the ipad 3's screen being bigger than the iphone or else the card sizes/UI will be too small or something. And the other thing is that there are plenty of games on ios that won't be able to be played on previous generations of devices due to the technical limitations! But what is also interesting to me though is that there are even more games than PTCG that are ipad exclusive that cannot be played on the iphone!

http://www.hongkiat.com/blog/top-ipad-hd-games/

The point that I am trying to get across is that not even apple really does this whole "games will work on all devices" thing because there are games better suited to the ipad due to its larger screen than the iphone! And of course the whole, iphone 6 being stronger than the iphone 4 so a high graphiced game on the iphone 6 will not work on the iphone 4! But anyway, we are getting a bit off topic so I will let you have the last word

I loved the GBC game so it would be nice if they made a updated game for 3DS.

The GB screen is smaller than both and had the first TCG game lol of course that's not a reason. And a Nintendo platform wouldn't get a hardware update in power yearly like idevices do, so those limitation wouldn't effect a unified platform gaming device at all. The point is that devs have all the tools to make a single game work on all platforms without breaking a sweat, which is true on iOS and Android. So yes, Apple does do the whole "games can work on all devices."

They probably will eventually, maybe for mobile :/