Frozen can't work in the TCG because of how it works in the games - it completely incapacitates you. The 75% Speed reduction that the game's version of paralysis brings is replaced by a turn of being unable to attack or retreat. You could argue the same for Frozen, but then it would be identical to Paralyzed - there'd be no point to adding it.
You can't increase the delay of attack (say, two turns instead of 1, like Paralyzed), because then it gets broken; odds are you will flip heads enough times to freeze the opponent so much that they will never get to attack.
You can remove the ability to attack altogether, demonstrated by
Neo Genesis Piloswine (whose attack, interestingly enough, is called Freeze). This is the only card in existence that removes a card's ability to attack permanently, providing the target remains in the Active spot. I think this would be a rather broken effect if it were used on current cards.
There's also the fact that you can turn a card so many ways. Remember, we are supposed to turn it to either side for Asleep and Paralyzed, and upside down for Confused. (This is one reason why you can't be Asleep and Paralyzed at the same time.) You could argue that you could make Frozen counters, but what would you do with them?
Oh, I've got an idea, what about a damage reduction effect? Similar to Metal Energy, a Frozen Pokémon does 10 less damage with its attacks (before applying Weakness and Resistance).
Eh. I don't think we'll ever see Frozen, and to be honest I'm totally fine with that. If a Frozen condition is ever invented, it would certainly be unique, I can tell you that right now.
