There are at least two main – mostly revenue based – reasons:

1. Google’s core business, search, continues to lose steam with a key metric, cost-per-click, falling as mobile app usage skyrocketed.

2. … reasons why Google has renamed itself Alphabet…: It positions the tech company to expand into health care, which could be very healthy for its long-term fortunes… Google’s revenues are extremely dependent on its relationship with Apple — and there are questions over what the Search business model will look like as mobile devices evolve. Meanwhile, health care is a multi-trillion-dollar business. Health care will stay a multi-trillion-dollar business for years to come… In the past year alone, the company has teased a pill that would detect cancer, debuted a plan to map all the biomarkers in the human body, and … half-dozen other wild inventions that could change health care forever… shifting from Google to Alphabet could catalyze a new wave of [health care] product development.
Read more: http://www.forbes.com/sites/dandiamond/2015/08/11/google-is-now-alphabet-and-it-could-spell-big-things-for-healthcare/

See also: Larry Page: ” we are creating a new company, called Alphabet”