Microsoft stopped supporting Windows XP on April 8, 2014. So computers running Windows XP haven’t received security patches in over a year and are facing a number of critical security vulnerabilities. But millions of people are still using Windows XP every day. Google wants those people to have the option to use a browser that’s up-to-date and as safe as possible. That's why Google has decided to extend the support for Windows XP through the end of 2015 by releasing security updates.
Source: Chrome Blog