Once they knew they had a secret to keep Bill and Monica were perfectly willing to lie under oath about it and would have presumably taken it to the grave had Lewinsky not previously confided in Tripp who recorded subsequent phone conversations about it, a rookie mistake on Lewinsky's part, she didn't know what she had gotten caught up in at that time.

Edit: I'm not trying to be a lightening rod for jerks who are looking for a tin foil hat to dunk on. I'm just trying to add context.