With the introduction of Alphabet, the parent and a new home to Google and all its products, the search engine giant company Google begun change. Of course Google changed the lifestyle of the people through its revolutionary products like Search, Gmail, Youtube, Google Car, Google Glass, Android, Google Wallet and so on. The change is in the logo has been a change to reflect all the change that has been going on.
For more details on Logo
Logo History: https://www.google.com/search?q=Google%20logo%20history
Google, evolved: Official Blog | Youtube Video
Google doodle: https://www.google.com/doodles/googles-new-logo
Google Logo Features: https://design.google.com/articles/evolving-the-google-identity/