“I hope you have not been leading a double life, pretending to be wicked and being good all the time. That would be hypocrisy.”

in the act "The Importance of Being Earnest"
why did Lady Bracknell said this?

Lady Bracknell said this because she is questioning the sincerity and honesty of the character in the play, particularly in regards to their behavior and intentions. She is cautioning against being fake or deceitful, as pretending to be one thing while actually being another is seen as hypocritical and false. Lady Bracknell values authenticity and honesty, and is reminding the character to stay true to themselves and not lead a double life.