It has been suggested Darcy was 'not entirely himself' when he wrote that letter. I suppose Lizzy's words cut him to the quick and he was determined to be discharged of the worst accusation, namely injustice to Wickham yet that also entwines his sister's secret. Now that you mention it, it does appear strange. Darcy knew Mrs Bennet for a gossip, and although Lizzy is discreet, if Mrs B. chanced upon the letter for example...! I can only think a letter was the only way JA could devise for Lizzy and the reader tofind out the truth of the particulars concerning Wickham. Darcy commented Lizzy took 'uncommon interest' in Wickham. I wonder if Darcy wanted to caution Lizzy lest she suffer some fate by George Wickham.