icowden
Pharaoh
Why is it that MPs can't say that particular truth in parliament?
All MPs are honourable. Therefore they would never lie to Parliament as that would be dishonourable. Thusly to call an MP a liar in the house, would be to dishonour them.
Also as @secretsqirrel said - if it were permitted then they would just call each other liars all day and we'd never get anywhere. It also prevents the hurling of baseless abuse as you cannot be sued for defamation for what you say in the house.
MPs that are significantly brighter than 30p Lee and Sultana find creative ways around the restriction. Churchill accused an opponent of terminlogical inexactitude. Dennis Skinner once stated that "half the Tory MPs are crooks". He was asked to withdraw the comment and said "Half the Tory MPs are not crooks.". he didn't get away with that one but you have to applaud it.