With all the changes for Google requiring mobile friendly web sites in order to be picked up in its searches they must now conform to their own guidelines and opted for an updated logo for mobile friendly viewing.
...On the day the logo was unveiled, Google shared words that echoed this
sentiment by stating, “We think we’ve taken the best of Google (simple,
uncluttered, colorful, friendly), and recast it not just for the Google
of today, but for the Google of the future.”...
The New Google Logo: Does it Tell the Future? - SiteProNews