Paraphrasing from commentary in front of P&P-the novel, I don't remember which edition, these two selfish people's actions had very dire consequences for Lydia and the Bennet family, in accordance to their times.

At that time, everyone found out that Lydia ran off with Wickham; that branded her a fallen woman, lost of her most valuable asset, her virtue. She and her family would be disgraced, shamed and ostracized by polite society and the community. The only solution for the scandal would be for Lydia to get married. However, when it was later determined that the couple didn't travel to Gretna Green at all, but to London, the Bennet family's fears worsened; without a marriage, the whole family would face irreparable damage to their reputation, which will cost all the Bennet daughters every chance of making honorable, decent marriages.