Why do hell would they change their name? Google is like the homepage of the Internet
Two reasons.
1. The EU is trying to break up the company like they tried to do to Microsoft. Google simply did it in a way that still allows them to operate.
2. The Google founders realize now that the Google brand is limiting their company. They have much bigger plans in the future with self driving cars, AI, and Glass that all have nothing to do with search, Android, YouTube, etc. Alphabet means that Google can expand into separate brands with their own focus under one parent company.
Google is basically turning themselves into the American version of Samsung if you look at their business.