The architecture for a computer program is very important since everything else in that program relies on it. Computer architecture is very similar to constructing a physical building. The program's architecture is the concrete foundation that you build on. If the foundation is cracked or broken, then you have a BIG problem. (I hope you understand that a program's architecture is very important even though you cannot see it.)
Forge's architecture restricts which cards are supported. Some cards cannot be added because the architecture isn't flexible enough. Cards like planeswalkers aren't really supported but were added by using elaborate hacks. The same thing applies to cards like Hypnotic Specter and Glorious Anthem. Even activated abilities that tap a card like Royal Assassin have to be "hacked" a little.
All of these "little hacks" add up and causes something later that "should work", not to work. Protection was a major update that required all of the targeting code for all of the cards to be changed. I didn't think about protection when I was creating Forge so all of the cards have to be changed one at a time in order to support protection.
Dennis, Rob, Zerker and others from the forums have donated weeks of their time toward Forge. And although I created Forge, these are the guys that keep it going.