Why? When I play one, I'm definitely going sword and board. I'll do it as a Defender with a secondary Controller aspect. That's going to mean maxing the AC and forgetting about being a damage machine. Maybe I need to look it over again, but what's the benefit of using a reach weapon?
The warden marks every enemy adjacent to him, and then, if they attack his allies, he hits them with an incredibly vicious attack that is a weapon attack against reflex, AND which causes them to grant combat advantage to his allies for the next round.
Unfortunately he possesses no intrinsic ability to stop his enemies from shifting.
So an adjacent, marked enemy can shift one space away, and attack his allies. The warden can't use his awesome, incredibly good retaliation attack. He has to use his secondary ranged immediate reaction power, that pulls the enemy one space and immobilizes him for the rest of the round.
If he has a reach weapon, his incredibly awesome attack lets him strike the foe even when its one space away.
So a warden has to make a bit of a decision. If he goes Sword and Board, he can have a decent armor class. But his enemies will frequently ignore his marking power and just shift away from him, suffering only a -2 to attack, and a pull of one space. If he uses a reach weapon, he gains massively in his ability to force enemies to fight him, and to leave his allies alone. But he loses the use of a shield. And with his armor class as low as it is, that hurts.
There are things you can do to make the sword and board warden better at blocking enemies. A lot of warden powers create difficult terrain near the warden, which prevents shifting. One of the level 1 Forms creates difficult terrain for enemies out two spaces from the warden, and some encounter powers are similar for shorter periods of time. But the reach weapon wielding warden doesn't have to worry about those things.