The reason Wales and Ireland 'choke' at World Cups is because they value the 6N Grand Slam over it.....theres a reason why France and clearly England have better success in the World Cup.
England barely celebrate 6N Grand Slams, it's just a tidy win, they target the world cup. Wales and Ireland seemingly still fear it......the pure proof is in Wales epic bottle job vs France in 2011, and Irelands vs Argentina in 2015......sure, England and France have flopped at WC's themselves, but they've also overachieved at them.