They are able to be copied by hackers and then used illigally and cheaply. The equipment to do this can be as low as $25. Source
EMV is the technology that CC's, Debit Cards or any magnetic card uses that makes them also chip enabled (as well as the machines that accept these types of "smart" cards). Stands for Europay, Mastercard and Visa (the three companies that first created this standard). Source
EMV cards are a chip and pin authentication, and provide higher security levels becuase they are less able to be hacked and copied, and also more control of "offline" CC transaction approvals. Chip and pin cards are authenticated through the processing terminal and the card's host system instead of a visual inspection of the CC holders signature and verification on the back of the card. Source
Because it was going to be a lot of work, and a lot of plastic to replace (not only the cards but also every terminal in use in the US). Source