Moores Law - More power in less space = more free space
DCs were built when server kit was physically big, power hungry, required lots of cooling. Now server kit is shrinking in size, power efficeient and requires less cooling, so they have lots of empty racks to fill.
Makes sense to offer a co-lo option, rather than building your own DC. All the benefits of someone else running the physical security, power & cooling etc but with all the benefits own running your own servers (instead of the Cloud aka someone else's servers).
Biting the hand that feeds IT
