Maybe it has to do with the fact that people aren't used to giving money to Google where they are more likely to have a trusted customer-vendor relationship with Amazon.

That being said Google is a well-known and trusted brand so I think something else might be at play here psychologically. Perhaps the public perception of Google's products as being "free" (to them) somehow increases the barrier to becoming a paid customer.

I have not an ounce of evidence to indicate that but I think it could be an interesting topic with big implications for the startup community.