My first guess would be that Google has a lot more cool things to do with its datacenters than Amazon, so up to now any extra capacity was used for a new produc

My first guess would be that Google has a lot more cool things to do with its datacenters than Amazon, so up to now any extra capacity was used for a new product/project. Only now did they have enough spare to start leasing it to outsiders (which either says something about Moore's Law, or about a decline in the rate of innovation at Google).