For me it was because I was an IT manager at the time and was trying to upgrade from a crappy onsite Groupwise email server, and Office 365 and Google for Business were both competing for businesses. When we did the trials of both it was no contest, you got more for what you paid with through Microsoft, plus they actually answered the phone and provided support for businesses. Google for Business had less features, their support was online only, and it took them forever to respond to us.
It was also an easy sell. We could pay $500 a month for our 100 employees, and could change those licenses on a monthly basis, plus we would get the newest version of Office for all of the employees who needed. The other option was that we could pay nearly 12k upfront for a new server and Exchange licensing, as well as another 13k for Office 2013 licenses for the 35 people who needed. Then we would have had to upgrade in 3-5 years and go through a painful migration process. The convenience, customer support, and pricing were all wins.
As soon as I saw what they were doing with Office 365 I put like half of my 401k into Microsoft. It's been a pretty good decision