Starting April 2016, Chrome will no longer be updated on Windows XP, and Windows Vista, as well as OS X 10.6, 10.7, and 10.8. Google says these platforms are no longer being officially supported by Microsoft and Apple, and it makes sense for users to update to newer versions of these operating systems to receive the latest security updates.
To be clear, Chrome will still work on all of these platform, but won't be getting any further updates.
If it's not bad enough to have an out of date operating system, it's worse to have an outdated browser that does not have the latest security patches.