Why services? Because on-premises installations become 'legacy systems'
There's a reason why so many things are moving to "as a service" status. A service is centralized and can be frequently updated by its owner. A dozen changes can occur to a service in the time span that it takes to convince an IT manager to make a capital expense. Once the IT manager has made the purchase, the first thing the business want to do is customize what he's purchased. That means it's a struggle for him to update the system, when the vendor gets around to issuing the new version. David Linthicum once remarked to me, when I asked when does a new, on-premises application become a legacy system: "Three minutes after it's installed." These issues don't arise with services, which come from a source under the control of the vendor.