The feature has been available to tablet users for months
Google is continuing to trickle down ideas from the tablet and mobile sites into the main desktop ones. On the one hand, it's one way to test new features with a smaller audience before rolling them out on sites as big as Google Search; on the other, it's a clear indication of where Google's focus is at right now.The latest such example is a new weather onebox, or maybe we should just start calling them "cards," which is now live in Google Search.
All you have to do is search for the weather at any location and you'll notice the new design. It bares the now unmistakable Google look, introduced with Google+, and should look very familiar to tablet users since they've been seeing it for many months now.
Still, Google has only now enabled it on the desktop though not in all browsers, the reason why it took it so long perhaps.
In the mobile space, it only has to test features with WebKit-based browsers which tend to be more up to date on web standards and technologies than desktop browsers.