I received a new card in the mail recently. My existing card wasn't due to expire until almost two years from now. The letter that accompanied the card makes it quite clear that the card was replaced for security purposes:
...You are receiving this card as your account number and possibly other information regarding your credit card account may have been obtained by an unauthorized third party...
The new card has a different expiration date (but only one month later than the old card) and a new 3-digit CVV2 code. The 16-digit account number on the front of the card did not change. The letter states that the PIN did not change either.
Both the old and new cards have chips in them. This card is issued by a well-known bank in the United States. It is a credit card and not a debit card.
What are some reasons why the bank would have replaced the card without issuing a new account number?