It takes a considerable amount of time to edit the game.
It's not exactly a "copy and paste" thing.
I'll try to simplify it for you, despite having medium knowledge on game coding, I'll make an assumption on how the process of the card implementation system works:
1. Card effect needs to be compatible with game, editing may be required.
2. Mass Recoding is needed.
3. Code checking.
4. Game error testing.
5. Fixes In error.
6. Multiple Testing, again.
7. Fix again.
8. Input the code into the actual internet server game
9. Check for errors again
10. If there is error, repeat step 6 and onwards.
11. Inform Public
12. Public plays with new version
13. If there is Feedback on bugs, needs confirmation
14. If Bug /error feedback was officially correct, repeat step 6 and onwards.
There may be even more steps than that.
You have to also consider that Zanz and his team have other life issues to attend to, so add that to the time consumed, and you'll see exactly why it takes a while to make additions to the game.