Narita is slot limited because it has just one runway, they have tried for years to get a second one, but the farmers in the area have balked. Great airport though, one of the few big ones that still has an outside observation deck. Heathrow is slotted because of space, gates and number of flights, its a big airport and has lots of movements.
It's about time Japanese consumers were able to buy rice on the world market. Texas or California rice costs a tenth what Japanese rice costs. If Japanese farmers weren't coddled so much, there wouldn't be any problem getting land for an additional runway at Narita.